You took the first strong step by realizing the issue and breaking up with him. You are not responsible for his decision to use, although he may find reasons to blame you. He has to do that, he can't blame himself. That is part of the disease. You didn't cause it, can't control it, and can't cure it. You can only take care of yourself. Trust me, he is with a new partner only because for now that person is likely not getting the situation. So he can have fun and not feel guilty. That is not a healthy relationship. You are much healthier on your own.
Like others have said, be prepared for when he contacts you again. Do the work now, know your boundaries and stand firm. You will find a lot of support here.