Hi lostMyWind, well I can completely understand all that causing the way you're feeling, and having to cope with the situation for so long as well. Are you in touch with any "mutual" friends or is there a member of her family you could talk to who could do some mediation for you??
I mean some things can't be undone (and at least you've cut personal ties with her) but if you could at least keep the contact going with your children.............And you are going to have some rights in relation to your children, some that are going to be hard to overturn.
Don't know whether any of these links may help, but.......:
https://fathers4kids.com/
List of Fathers' Rights Organizations in the U.S.
And all the support you can get, hey? So, is your girlfriend supportive to you in all of this? Maybe you could let her know some things she could do to help you in the way you're feeling/and in what's going on for you
But as much as you can try to focus on you as well, hey? It's easy to see how all this could almost consume you, whether you're constantly thinking about it or not. If you can try to do something/s for you, to try and push away the depression just a little, or even get some very brief escape from it...........whether that's pushing yourself to go out and do something, taking a break from "routine" or just taking a little more "time out"...............something that works just a bit more for you.

Alison