I have no idea what to say other than he loves you, that's why he didn't stay single. I care, I am sending you a virtual hug and blessings beyond measure. Please hang in there.

You are not a horrible person. He saw things one way, you saw them another, and they didn't line up but neither of you could walk away.

Yes, he should have stayed single if he was going to be so involved with BM and be at his kids' beck and call (or he should have found a partner that was okay with taking a backseat). You should have moved on long ago when you realized that he could not be the partner that you wanted, without you forcing his hand. Hindsight is 20/20. You wanted the relationship to work so you hung in there, trying to create a balance. But, it went from one kind of dysfunction to another, which was beyond your control.

Nobody knows if he would have a better bond with his kids had he kept on the path that he was on. Nobody knows if his kids would like him better or if BM would have included him in college visits.

No point in going there.

Your SO did have a choice and he chose you.

Set the boundaries and standards of behavior and performance, enforce them, and let the cards fall where they may with those who pull shit like BM.

You did not make BM who and what she is. That, is entirely on her.

My dad recently shared a philosophy that made an impression on me.

If you look in your rear view mirror, that is the past. If you look out of the side windows, that is now. If you look out of the windshield, that is the future.  Live in the now with a view of the future.

What is in your rear view mirror you have zero control over. It is in the past.  Learn from it and use those lessons to make the future better.  What BM has done or will do, is on her.

Apply the appropriate consequences for her actions if those actions violate the standards of behavior and standards of performance you demand.

KISS. Do not what if the joy out of your now or your tomorrow.  Make decisions with the best information available when you make the decision.  Adjust going forward if necessary.  We only know what we know. "What if" can cause analysis paralysis and destroy your joy.  Whether that what if is rear facing or future facing.

Don't do that.

You chose him. He chose you. You both chose to make a life and love for the ages together. No regrets.  BM has nothing but his past. You are his now and his tomorrow.  Let her wallow in her noxious cess pool of the shit that she is.

DH often said to me, "Know that this is not because of you. It's not about you. All of this would have happened with ANY woman I married. No one I dated was ever accepted. This isn't about you; it's her [BM]." 
Repeat those words to yourself. They are true. 

I guess in these situations the men could choose to be lifelong puppets and remain married but not  married to those they divorced. 
 

He made his choice. If I was a single parent I personally would choose my kids and delay or restrict my personal love life until I have free time and my obligations to my children ended.

I always told my husband that his children should be his priority and he should do wtv it takes to have a good relationship with the exes. I never complained about money or time or services he did except when it would impact me or involve me. Even so, they all blame me for the downfall of the family because I was too "strict" According to them when they would visit and I fostered a hostile environment (turning off the wifi, making them do chores and asking about homework) that made them drop out of school

After I left my husband, suddenly his relationship with his children fell apart without my involvement...I realised I served as a buffer and scapegoat for them to distract from the real problems (being bums who have no desire to be productive on society and want dad and mom to cater to them)...

You aren't the problem, your husband is. If he wants a relationship with his children, he is fully aware of what to do, but he knows that he won't win. Because he can choose to do all the stuff you listed and they would still turn on him whether you are in the picture or not. Your presence is just easier for them because you serve as the scapegoat for the failed family

Being a stepmom comes with a load of guilt, mostly unearned.  We are the convenient target when kids are unhappy about their parents divorcing and moving on.  Sometimes, we truly ARE guilty, I know I failed to understand the transition issues and was often quick to lay blame on the SKs when my bios were equally at fault.  I think it's all part of being human - the SKs and BM probably misunderstood my role and I failed to understand where they were coming from.

All I can say, and you can probably say the same thing, is that I tried my best every day, with what I knew then, and with my own physical, mental and emotional llimitations.  I know I failed sometimes and I'm sorry.  But, we are all human.

I try to forgive myself for any guilt and I hope you do, too.

Divorce is an unnatural situation and doubly hard for stepmoms.   It used to be that the BM had to assume room temperature before stepmom "stepped" in.  This is the secret to The Brady Bunch's happiness.

If SO lived in a cardboard box . And gave every penny he has to BM.  Her life would be great. But no SK wouid visit because he lived in a cardboard box. 
ITs not you.  It would be anybody he was in a better relationship then BM.  Certain things happen.  

I've had similar thoughts over the years, even though my skids were adults (chronologically) when we married. If I had continued to be invisible on command, turn over my money, allow DH to back out of promises to me in favor of his kids, watch him jump ever higher to please them, then maybe they'd not be estranged now. Maybe. But surely I'd not be in the picture, which would be great for them, I'd have been fine after the broken heart healed, and DH could still be their fool. Maybe another woman would be in place to take the abuse.

There are many possible paths to walk. I've made good and bad decisions in my marriage and in my life. I can't be responsible for the decisions other people make, and at any point DH could have told me that his kids' commands were more important to him than being my partner. And there would still never be enough money or worship handed to them , with or without me. 

I've felt a lot of guilt as a SM, but have recently challenged it more. I've been thinking along similar lines as you, but have come to a different conclusion. Basically, I'm realizing to what extent dh, SS, and BM prioritize their needs, preferences, and narratives, leaving mine to come a far distant after thought. For example, SS has a schedule of visitation based on their prior CO (SS is 19). It serves to give BM a break from SS and the dog, it allows DH to spend time with SS, and SS prefers to stay here, so he'll stay here as much as he can if he's allowed. And what about my needs? It seems too much to ask for everyone to inconvenience themselves for little ole me. Even I thought that way. But I am a human being in DH's life, and have a significant place in it, Part of being human is to have needs and preferences; I can't eradicate my human nature. It's odd to say, but I think our life together had been premised on the assumption that I should have zero needs of my own, but that's impossible. Dh used to get angry if I dared to make even a minor request like not extending the visitation beyond what was agreed to, or no, I'd prefer that SS does not come any earlier than he usually does. If I asked, dh would get angry and accuse me of 'hating SS'. So I go along with what everyone wants or I'm evil?

I feel like BM and SS are of the belief that since they are the original family, they have more of a priority and say than me. And DH probably thinks or thought that way too, mostly because he wants to protect the interests of his child. But dh is coming around to the fact that it is not a betrayal to his child to also include me in his sphere of concern. If SS protests or gets angry if I get my way sometimes, that is unreasonable. That's not even a good way to be as a person. Rather than cave to those bad impulses, the child should be guided to cultivate empathy and patience. It also models a healthy relationship that would be good for SS' future life with a SO.

challenges together.  Since we are SPartners in a blended family that makes a notable part of our role to help raise SKids, address challenges with the failed family opposition when they step out of line, and to set an example for all kids in the blend (yours, mine, ours) of a quality respectful adult partnership that raised children with standards of behavior and standards of performance.

Key to success is that the marriage and the partners prioritizing each other are the hill to die on and defend from toxicity at all costs.

IMHO this is what is in the best interest of everyone in the situation primarily kids and even what is in the best interest of the overstepping opposition.  If they are kept in their lane and forced into compliance with the CO, it is best for them and everyone else.