In a relationship every party holds some responsibility to their partner(s). If one has not found some balance to mental health issues through medication, then that responsibility will feel like a noose around their neck. You can ask someone to relax, but of they have someone to depend on then they will always put themselves through more pain than they can bear for the sake of others.
Owning up to one's problems and creating a plan for a healthy future with a therapist is also very draining and stressful.
The feeling of one's own body working against them like through hangovers and detoxing adds another layer of difficult stress.

All of that together is more than most people can handle. He doesn't want to lose you, but right now he is under so many sources of stress that he has lost himself. You make him happier, but his lapses show that your relationship has not made him stronger. Not strong enough to handle all three.

Right now when his drug problem rears it's ugly head your relationship cracks against you like a whip. The problem's head will become far uglier and more painful the longer he goes without successfully treating this. Perhaps he has grown stronger through your relationship and the opportunity to share his past pain with you. I believe that he cannot gather the strength to resist the pain of alcohol while you are there to support him and soften it's blows. You should not leave him for his sake, but for your own too.

That pain you feel when he broke your trust? It matters. The discomfort you feel with your partner drinking? It matters. Don't make yourself go through such emotional pain.

Maybe my recommendation shows that I am immature because it is selfish. What I know is that alcoholism over time changes people for the worse. There's a reason violence against one's partner and alcohol are so commonly linked. After bashing himself for long enough over his failures anyone would lash out at others. If you support him and he continues to fail... you might have to send him to jail someday. This is an adult problem that may lead to years of waking up after painful nightmares, or it could end up with two healthy adults that were able to care about their own health.