Just because a situation isn't suited to you.. doesn't make you a bad person.. but staying and trying to discourage a parent from having a relationship or supporting their child.. is not right.. and it might make you a "selfish" person if you put your desire to have a relationship before the needs of other people.

I don't have kids of my own.. and becoming a step parent to two girls who were 5 and 9.. was challenging to say the least.  It's not for the faint of heart.  It's not for those who need to have a high degree of control and certainty in their lives.. and it's not for those that can't get past the fact that their partner had a relationship before them.. and can't accept the financial and moral obligation to be present in their child's life.

But.. it also falls on the partner to raise kids who will behave respectly to  his new partner.. for setting reasonable boundaries and expectations for his kids.. for how his family treats his new partner..   He may have little control over a dramatic EX.. but it's also his job to minimize the impact on his new partner.. that did not create kids with that ex.

A new partner is not always THE priority.. his primary obligation is to raise his minor children.. but their wants don't overpower the partner's needs.. everyone in the family should be important.. and be a priority.. although.. sometimes the needs of some will take precedence.

Without more of a window into why you feel you are the bad guy.. why it doesn't work.. it's tough to give a full read.. everything above is just general reflections.

I believe kids should enhance your life, not consume it, but in todays society it’s so kid centric that couples lose themselves in the children and forget they are partners.  I know for me having a weak ineffective partner who catered to his daughter to the determent of our marriage has left lasting scars that don’t heal.  Once his daughter aged out the problems persisted, it just solidified that he is a shit partner who has no clue how to be in a functional relationship.

If i'm not mistaken, you are a widow after a long marriage without kids. That's the life you want. A guy with teens, loose boundaries, and a dysfunctional ex won't make you happy. TBH, it probably won't make anyone happy. I have kids of my own and an ex, but my boundaries don't align with someone with a total lack of them. Plus when they start hiding things, it only makes it worse. You are not THE problem. Wanting different things from your partner and trying to hammer a square peg into a round hole is A problem, but the blame isn't all or nothing. In the end, it doesn't matter who is to blame. It works or it doesn't. 

Do not brutalize yourself.  It is his baggage, not yours.

While his past cannot be erased, if he is managing the baggage from his past effectively, it will not overly interfere in your life and the life you and he share.

Can you make adjustments? Certainly. 

But his past is his and you did not create it. He did.  

Take care of you.

Living your best life should be your focus. Living well is also... the best revenge. 

So, stop blaming yourself for his baggage and enjoy living your best life ... and the related revenge.

IMHO, even in intact marriages, the marriage and partners have to be the priority above all else. Always.  Kids are the top adult responsibility. Not the priority. Kids benefit from a prioritized adult relationship. It sets the example. Kids need that example. This mistake is at the heart of many failed families and marriages, first... and subsequent.  Parents who sniff the asses of their children for some reason never seem to stop. Ever.  Which can end even a long term initial marriage after kids should be viable adults.  The first time I heard a person say that their kids were far more important than their mate (the father of the children) and he would be gone if he ever through he was more important than the kids, I was stunned.  It was a coworker when I was in my mid 20s.  Another coworker saw the look on my face and said to me "Your parents have been married for almost 30 years.  This is different."  Which I also did not understand.  Why be married if your mate is not your priority and you theirs?  Not my paradigm for sure.

The example we were raised with was the exact opposite.  Mom and dad were the priority for each other, as was their marriage.  It was clear that we were to never put our parents in the position of choosing us over each other or their marriage and that doing that would be a near fatal error.

I had the same perspective in my first marriage. My XW, did not. Fortunately, we had no children.  DW and I, are on the same page. and always have been. Though me choosing from day one to be dad to her son, mitigated much of any potential related drama.

Your likely STBX failed once, and is doing it again.  At least the second time because he has prioritized his failed family baggage over his marriage/mate.  That... is on him.  Not you.  You have been banging the marriage/mate priority drum the whole time. He... is deaf to all but the flatulence emitting from his kid's ass. He is incapable of being a quality partner.  Nor can he smell the roses of life. For obvious reasons.  He is a failed man, a failed partner, and sadly, he is failing as a father.  The parent model he is providing to his son will in all likelihood result in similar adult failures for the kid.

Be kind to yourself.

Are so hard. You want to control your life. Not have BM and SK controlling your time.  DH has to understand if he wants to talk to his kids, the world doesn't stop for him .  If he wants that he shouldn't be in a relationship.  You in your mind as some point understood that SK will be over X days and Y time.  Not nights watching sports. Bor phone calls in the middle of tv time