Not to take away from how destroyed you feel, especially after being together for a few years, but my first thought reading this was he didn’t leave you for her, he left you because he’s losing control of himself with the drugs. You’d be a strong reminder that he’s not worthy of good things and it’s part of his spiral downwards. I don’t think it changes that you need a healthy distance from him if he’s treating you badly, your first job should be taking care of yourself. But the me vs. her isn’t as much a thing here as the drugs are.