I'm not trying to dissuade you but I feel every married person on the planet has seriously thought of splitting at least one time.  For step-parents, it's probably 10,000 times.  I thought seriously about leaving for hours every day at one period.  Actually, there were several periods like that, some more intense than others.

Wishing you the best, it's so hard sometimes.

That's my suggestion. You need someone unbiased to help you through this. Someone who has experience in High Conflict Step Hell. LIke JRI said, I feel every married person has thought splitting up...especially in Step Hell. But I'd strongly consider counseling before making such a major decision.

I understand your hurt. I invested 12 years into individuals....invested my time, heart, sweat, tears, etc....and got nothing in return. Today, I am completely cut off from DHs kids and now his grandkids. Being shunned from DHs grand children was deeply hurtful to me (and how it happened was humiliating). Took me 3 years of healing...working with a counselor, doing group studies and reading a lot of books. I'm better today. Won't say it still doesn't hurt sometimes but I'm able to process it now with techniques I learned over the years. And my marriage is fine, also. DH did come with me to many counseling appointments...while working on me, we also worked on "us."  Best to you.

There is a pattern on here of sweet stepkids becoming angry, vengeful teens, especially when they have a high conflict mom and a new half sibling. None of this has anything to do with you, period. Yes, you made a mistake getting close to BM, but it was a well-intentioned mistake and you had no idea any of this would happen. You did not cause any of this, it's BM's pathology being expressed through the kids. 
 

Step back, focus on your DD and DH, and let them go for now. Feel your grief and honor it, you've lost people you loved in an awful way. Keep BM blocked and let your DH try to rebuild his relationship with his older daughters outside the home. 

Early last year, I had to grieve the relationship that I thought I had with SD115 backstabber/munchkin. She activated her mother Toxic Troll against me and it has changed our relationship. I grieved it and vented here, so I understand how you feel. It took a very long time because nothing was addressed and everyone acted like nothing had happened. Some arguments happened and now I call things out as I see them. I am refocused on myself. This new year I will be continuing that track.

For you, focus on your marriage and your child. Grieve the relationship you had hoped to have and thought to have had. And move forward knowing you did your best with them.

I dealt with the potential hurt by focusing on behaviors rather than the feelings that can be so painful.  If you focus on the behaviors and apply consequences you are addressing choices by the Skid. By making that behavioral choice, they are choosing the consequence.

That prevents the heart break.  It delivers a very strong and real time cause and effect lesson that you repeat as many times as the kid chooses to fail to deliver to the behavioral and performance standards in the blended family/blended family home. 

No different than parenting in an intact initial family. At least when the parents involved are worth a shit.

Good luck.

Short Version: Got together with high school friend 26 years after both of our relationships coincidentally ended around the same time 6 years ago. We have a 5 year old together.

His 1st marriage now 24 year old SS and a 19 yeard old SD. His 2nd marriage a 18 year old SD, 16 year old SS and a 13 SS.... All pretty good kids!!!! He handles discipline pretty well, I dont get involved only butt in if involves or affects my 5 year old.

His first marriage kids grew up with their biomom out of state.. The SD decided she wanted to move in with us in 2018. She came that summer to visit and NEVER went back home. She attended high school and graduated. During this time her dad lost his job for about 1 1/2 year, 0 unemployment (long story). Her mom paid 0 for child support until like the last 7 months when biodad told biomom to pay!

I never agreed to having SD (mind you she grew up with her BM out if state) move in to my home, I purchased it as he lost his home/foreclosure ( baby mom drama). It is actually my second home as I left my biodaughter, 23 years old at my primary residence. I did not want to move my "boyfriend" in to my primary residence with my daughter!

Fast forward 3 years into senior year in H.S., SD starts complaining about me to her aunt. BD's sister. It all stemmed from her BM (out of state)... I had made it clear to dad that I prefer her to return back home once she graduated high school. Obviously she wanted to stay as she doesn't like her life with BM.. BM lives with her parents, oldest son and other family in one house. So a lot of adults in one home.. Here with us her 1/2 siblings visit on a week on / week off schedule. So pretty much just BD, me and the now 5 year old.

The BD would not support whining and wouid talk things out if there is an issue. She decided to complain to her aunt. Her aunt instead of coming to her brother, BD she takes all the whining at face value!!!  SD decided to return to live with her BM 2 weeks before graduating. SD and BD are not talking, some tension.

The aunt also has a son, same age graduating at the same time and asks to include her niece in the family celebration. 2 cousins graduation party, BD complies... Nothing wrong with celebrating their graduations from the same school together, right???

What was not informed to BD until Satyrday one day before his daughter's graduation was that BM (out of state) was and had been staying at his sister's house since Tuesday and was invited to the family gathering. Everyone in his family knew, including his other daughter one year younger (half sibling).

You can imagine how upset WE were... More the deception and lies were hurtful!! We both attended the celebration (his own daughters graduation). Neither of us have spoken to his sister since June.