Hello Betterdays
Welcome.
It is hard to deal with a break-up, but you have been together for two years and he says that he now feels happier than he did during those two years.
You broke up before and he gave the relationship another chance, but it sounds as if he is certain now that you belong apart.
You cannot force someone to stay with you ~ and, even if you could, you would not be happy, because both parties need to really want to be in a relationship for it to work.
You mention his drugs and your issues, so it sounds as if you both have other things to sort out.
He knows that you still care, if he needs you as a friend ~ there is little more that you can do, except hope that he sorts out his problems.
Can you get some counselling to help with yours?
Maybe, at some stage, it will be possible for you to be friends, but the wounds are still sore at present, so just take care of yourself and try to sort out your own problems.
Good luck!