Welcome to the family! That sounds like a lot to deal with, but you're in the right place.
I can tell you care a lot about him and want to help him. Unfortunately, I think you already know the answer to your question of whether it's possible to help someone who won't accept they have a problem. They have to accept it, and not just occasionally when they're upset, but consistently.
Do you rent or own the house together? Is he still living there while he takes "time to decide what to do," or has he moved out?
Even for those of us whose partners aren't addicted to drugs/alcohol, a lot of the principles are the same. You might have noticed there are a lot of articles and threads with helpful information. I'd suggest starting with some reading about codependency and how to protect yourself by stepping out of the rescuer role. Here is a good start:
Codependency and Codependent RelationshipsYou might also be able to get some in-person support from something like Codependents Anonymous (assuming you're in the UK,
www.coda-uk.org/).
None of this will help you decide what to do about your relationship, but it'll help you start to get yourself and your own life back. That usually makes it easier to see what you need to do next.
Hang in there! Please keep posting and telling us more as you feel comfortable. We're here to help, with everything from minor daily arguments to big ongoing problems.