This is a lot to unpack but good luck to you. I doubt you’ll be saying the same things five years down the line, and I think it’s horrific you believe that this is your first healthy relationship – with a partner in active addiction, who isn’t trying to recover, who you say regularly doesn’t care about you and cheats on you. That’s really frightening.
I know that’s harsh, but speaking from experience, nothing but tough love has ever worked for me when dealing with a partner with addiction. You need boundaries or they continue to do what they want, that’s how addiction works. They won’t stop until they’ve no choice left. And by you being there as that constant support, you’ll drive yourself insane and sacrifice everything you are for them. There’s meetings and help available for him if he wanted to change for you, is he going to them? Is he putting anything into this fight, or is it just you? This isn’t a healthy relationship no matter how much you might tell yourself it is, I’m sorry x