I met SD12 when she was 5. I wasn't a great stepdaughter, either, and my SM was never nurturing to me like she was her biological daughter.

My H is a Disney Dad and BM is a BPD, controlling psycho. I got way too involved and am trying to back track after 8 years. I have invested myself emotionally and am having a hard time stepping back. 

My SD12 has learned that lies and manipulation help her to get what she wants and neither parent can see it. I don't really have advice, just experience and heartbreak. I don't think I'll ever leave my husband after this long. I plan to let BM and him deal with SD while I work on detaching from all of their madness.

Message me if you need a friend.

I came into my SD's life when she was 5.  She and her BM and my DH were exactly how you describe.  By the time SD was 12, I wanted nothing to do with her.  She is now 14 and I am completely disengaged, even though she lives with us.  When she turns 18, she will be out of my house and I will see her as little as possbile.

With a toxic BM there is no chance of you ever having a good relationship with this child.  None.  That me say that again.  No chance.  Multiply that by two since your step is a girl.  And the BM is not going to suddenly welcome and respect you once you marry.

Personality is set by age 7.  SD is genetically predisposed to have the same mental health issues her mother has.  Compounding that fact she has received little to no discipline up until now, and is being actively told she is not required to cooperate.  She has been given too much power and thinks she is on the same level as the adults.  Even if your DH underwent a magical transformation, which he can't, it is still too late to change her.  You are facing a lifetime of Oppositonal Defiant Disorder.  Children like this are all about power and control, and when they have this personality, there is nothing you can do to change them.

Here are some links on ODD children:  https://www.empoweringparents.com/article-categories/child-behavior-prob...

No offence but why would you date a man with a 7-year-old and a godwoman

You stated in your post, "Part 2 is working on myself. As much as I don't want to be the step mom my step mother was, I am having issues wanting to bond with his daughter. I know her issues are not hers, they're a result of the destructive parenting her mother has given her, and the lack of parenting her father has provided her."

I commend you for beginning to realize that SP'ing is much more complicated than most people think.  You are launching into this role yourself now, and this gives you the ability to see issues from the other side, so to speak.  Ispofacto alluded to this above, but the reality is BM and bio-dad have the most say/ influence on how SM and SKs get along.  Yes, there are nasty SMs out there, just like there is nasty aunt, uncles, moms and dads.  But, for the most part, manipulative, controlling BM and weak, enabling DH = step hell.  And, there is very little any SP can do about that, other than maybe not bcoming one to begin with or disengaging after the fact.

I'd like you, if possible, to go a step further and realize that your own SM was probably put in the exact same position you seem to be going in now, and that is the more than likely the reason for any of your angst with SM vs. her personally.  It is important that society as a whole starts to see the bio-parents as responsible for a lot of this SM or step-dad vs. SK attitude that seems to be out there and so prevelant.  This is the only way things will change in the future and smooth transitions over for BOTH SMs and SKs.  BM's PAS'ing and DH's head-in-the-sand is detrimental to all.  And, without at least a strong DH, your future does not bode well.  A toxic BM usually means a toxic relationship for the entire family, including initial and blended.  None of this is fair, but it tends to be true.

The age of the bios getting a pass on their negative behavior/ all out war over a divorce, that probably happened years ago, needs to end.  The focus needs to be on getting everyone to move on and not on rehashing old wounds or using convenient scapegoats, such as SM or step-dad, to blame when things don't go right.  Anyway. . . . Best of luck to you and the decisions you need to make!

I'm going to add that when it comes to SP'ing and even potential future SP'ing, it is important that early on, the roles are not messed up.  A child is a child and to be treated like a child.  An adult is an adult.  An SO is an SO.  And, a married couple is a married couple.  Mom and dad are parents.  Exes are exes and they are no longer a couple.  They made the choice to give the coupling up when they divorced.

Some people actually think it is cute to let kids do things like pick seating for everyone, etc.  Once in great, great while, OK.  On a regular basis, never OK.  This is sending the message to the child that she/he gets to set the rules, and in the case of SP'ing, it can be even more detrimental and is not a minor issue.  The example you gave, "She demands adults sit in certain seating arrangements to purposely put herself between me and her father," sends the message loud and clear to SD that it is daddy and her, and then you--or, she is daddy's partner and you are the underling.  This is where a lot of this SK vs. SM crap gets started, at this early age, and sometimes even before SM comes along.  

Any woman (or man) who comes along later or lets this go on, that child is going to think of them as some sort of competitive equal vs. an adult whose role as dad's SO or wife they need to at least acknowledge and try to respect.  There should be no tit for tat.  You hear all the time my SM got this, but I didn't, or you'll hear some 30 year-old adult SK complain that SM gets to spend more time with dad than he does.  Hello!?  SM is dad's wife!!  She dang well better be spending more time with him.  I know my parents spend countless more hours together with each other than either one does with me.  

Or, you get DH saying things like, he doesn't want to be put in the middle.  Implying that SM and the SKs need to work it out among themselves, again, like they are some sort of equals.  In addition, SM's only backing is her DH, at least initially.  Yet, she is supposed to go toe-to-toe with CODs, BM, in-laws, and so on all on her own.  DH doesn't want to be put in the middle because it is easier for him to lay his children's angst on you vs. him stepping up to the plate and being a dad's dad to his own children.  On the other hand, these children are not yours and you're supposed to be able to handle them!? Whom is really being put in the middle here?  SMs and SKs don't need to be in competition.  They need to honor the roles.  You treat a spouse like a spouse and a child like a child.  Everyone should be aware of who dad's spouse is, and it is not his ex-/ BM. 

positioned to have a successful blended family relationship. You have experience as an SD that will be useful and valuable in your journey as an SM.

That said, I would like to give you a different perspective on something you said in your OP.  You indicated that the Skid should be the priority in this relationship that you have with your FDH. I completely disagree. The uncontested priority for each of you must be each other and the relationship between you.  It is that relationship that is at the heart of the blended family and that is THE priority.  Children on the other hand are the primary responsibility in the relationship.  Priority and responsibility are two very different things.

As equity life partners you and FDH are also equity parents to any children in mix regardless of kid biology. I would not be part of a marriage if this were not a foundational principal of the relationship.  Fortunately this is something my bride and I agreed  on from early in our relationship and it stood us in good stead while raising SS-25 since we met when he was 15mos old and married the week before he turned 2yo.  It worked well for us.

Whether SD-7 likes it or recognizes it or not ...  you are an authority figure.  If her behaviors are unacceptable then step up and parent. If your FDH doesn't like how you parent and discipline then he can step up and get it done before you have too.  If he doesn't step up then he can have your back and bite  his tongue until the two of you can discuss it in private.

The same applies to your FMIL.  If her behaviors and words are unacceptable then confront her on it.  If FDH does not like how you deal with his mother then he can step up and get that done before you have to as well.  My bride and my mom are BFFs (which makes my life more challenging let me tell ya) but they also respect each other very much and neither has difficulty having difficult conversations with the other.  I have rarely had to inject myself into my brides relationships with my mom or my dad. In fact... my parents like her far better than they like me.  *diablo* 

I think you are journeying down the right path.  Be confident, stand your ground, be assertive, and you and FDH make each other the priority.... and of course communicate. communicate, communicate.

Good luck.

that there are issues with your SO and his daughter, and for trying to educate yourself about step dynamics. Many women delude themselves during the courting period, and end up miserable later.

First Number One Most Important Thing That No One Told Me: You need to focus less on your SO's individual qualities, and vet the dynamic as a whole. You've already identified several serious points of dysfunction:

1. A weak dad who is afraid to parent.
2. A high-conflict BM.
3. An overly involved MIL who spoils and coddles skid.
4. A genetically compromised, spoilt, female​​​​​​ skid.

Countless divorces can be credited to any one of these criteria.

You, my friend, are up against a stacked deck of Post-divorce dysfunction. Do not marry this man unless you are prepared to accept and be miserable with the existing dynamic.

The only positive I can see is that the child is still young enough to modify her behavior. But that would require everyone involved to embrace change and a shared goal to stop ruining her. And you would be the catalyst for all this upheaval, which  would cause resentment and further discord. Basically, unless and until your SO has an epiphany and makes meaningful changes, things will only get worse.