I agree with Across. You sat on so much, for so long, and unfortunately have reached a place where it just wasn't possible to keep quiet and put up any longer. That has meant that you are having to deal with/confront loads of problems all at once.
If it was just ending your marriage to a bully, it would be tough, but you're having to deal with not only ongoing bullying, but the fact that you found out he was a liar, who had put your health at risk.
If it was just trying to help your DC adjust to the divorce, it would be tough, but you're having to deal with their inevitable hurt and confusion, and H dicking them about to get to you.
If it was just facing the fact that your mother is abusive and doesn't respect your boundaries or feelings, that would be hard, but she has chosen to make the whole thing harder for you.
On top of that, you're studying, and worrying about starting a new career.
Of course you feel you're not coping, but you really are. You just have such an enormous amount to cope with, that it must feel overwhelming. It will get better, you are getting stronger, even if you don't feel that way, and this will pass.
There are really only three people you have responsibility for, you, and your two DC (while they remain children), the rest of them can go hang.
Take your holiday, recharge your batteries, cry if you want/need to, and focus on that light at the end of the tunnel because it is getting closer.
And god your H is a sleaze bag, getting a friend to try and come on to you, yuk!