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 situatoin and doubly hard for stepmoms.