Interesting story. Your ex sounds like a right piece of work that has bent you over a barrel.
Don’t be so hard on yourself. You’ve clearly been a great dad to your kids and done your best. I say don’t suffer in silence. Sometimes you have to limit the damage done by people in order to survive and thrive going forward.
Don’t worry about other people, apart from your kids. Your ex will do whatever she is going to do with whoever she wants. It will be hard on you if you let it.
All you can do is your best and it sounds like you have done that. Therapy is great for clearing your head and helping you see the right path, there is no stigma in admitting that you’re only human, even if your kids think your superdad!