I'd like to join Sophie_M in welcoming you to the Forum which has may understanding and caring people, most having been through hard times.
Life certainly has thrown an awful lot at you, from children with problems to separation, and each time you have risen to the occasion, despite there being no manual, not knowing what to do, but succeeding in raising your children anyway. I admire your accomplishment and can see the love and concern that lives inside you.
Now you are at the stage where you have given all and see even more in front of you. There is a real temptation to assume that for the people you love you assume you are a bottomless well of strength, help and care. Sadly that is never the case. You can drain yourself dry and that stops you going on. Unfortunately rather than seeing the marvelous good you have accomplished you most likely feel guilt you cannot continue.
I know the feeling, but in different circumstances. My wife spent 9 months in hospital before passing away and by that time I too was completely drained -I even felt relief along with guilt, anger, loss and many more emotions.
I suspect that you are so exhausted you see things as simple black and white. For you to give your ex the main burden now does not mean it will always be that way - and even if it were as you recover you will be able to contribute more to them rather than only just holding on until you crash.
May I ask what your ex thinks of the idea?
While they might not want to live with their dad the way you put it is not that he is a bad father, simply has a different approach to you, something they would have to get used to. He would probably have to get used to being more flexible too in order to keep their relationship going well.
Trying to make the decision all on your own is very hard, and leaves huge doubts whatever you decide. Sophie_M's suggestions of who to talk with are good ones.
In addition do you have anyone, perhaps family or a friend, you can lean on, talk things over and gain fresh perspective?
You know you are welcome here anytime