Need some serious advice.

I do and I am female it's no different for men than it is for women. My advice is that he gets himself well and not focus at all on her. Once he's healthy he can reach out to her if he wants or start dating others. But he can't be happy or make anyone happy if he does not work on himself and stop PMO. The issue with saying totally give up is we don't know the whole back story and he has the right to feel what he feels.

 

It's too late. You don't need her help you can do this on your own and it's selfish and unfair to ask her for help. It appears it took her leaving for you to actually stop but you ignored her other requests, it's too late.

 

Man, be the best you you can be. With or without her.
Telling her "I can change" isn't going to help. Play it out in your mind, can you say those words without sounding like a whiny douche? If this is truly worth fighting for, admit you have made a mistake. Apologise. And say what you are doing about it. Ask for forgiveness and help.
And this may or may not work. But really, man, be the best you you can be. With or without her.