You described it perfectly, the highschool breakup x 100. My first husband was not good for my kids and I knew it, they were young, but it still hurt so much to make this decision. When I made my vows, I intended on keeping them, but sometimes your brain tells you, this isn't the way to live or the way to raise my children. Realizing it was over was the first time in my life I sobbed crying. I have only cried that hard 3 times in my entire life. It hurts to come to that conclusion and its like losing part of yourself, because you're so familiar with what life has been like for you up until now.
I am now remarried and I have 3 difficult children that now don't have their biomom as part of their life. Even though they are still difficult children, the confusion she caused and the instability is no longer there and they have improved. They still have many things to overcome, but now that they are in a household that is stable, if you could do direct comparisons to how they were and how they are, you'd see a huge difference.
My first husband, although not good as a full time dad, is fine as a part time dad. I think the kids live a more normal life now than they ever could have if we had stayed married.
My heart goes out to you for how hard it is right now. I never want to go through that pain again, it is such a hard and scary decision to make and you are constantly second guessing yourself with "what ifs". Hang in there, it will get better.