Thank you for being brave enough to post about your situation. I'm struggling with the relapse on an exbf, and I have a lot of empathy for where you are right now. Hopefully you can get a little distance from your BF be it moving out, asking him to move out, going somewhere for a night, or even going somewhere for a day. I'm finding, slowly, ever so slowly, that distance is what's helping me. I've also been reading the forum posts and going to meetings--perhaps there are AlAnon meetings where you live? Or if you're religious, a church or clergy person? Friends help as well. And though being alone too much can be detrimental, perhaps trying to love yourself through meditation and/or exercise or writing in a journal?
I hear you when you say, "I want nothing more than to trust him, to believe him." But you need to trust your gut. More than anything, it will tell you--and from your post, it has told you, what's happening. What I also hear is a deep yearning to trust someone, to believe someone. Trust his actions, you know what he's doing. Addiction is a destroyer of love and faith, true love and faith. I hope, for you, that you are able to find peace, and I believe that someday you will find a man who is WORTHY of your trust and belief. I read somewhere that when we don't say goodbye to what we know/who we know is not good for us, we're turning away all the lovers/boyfriends/husband/loves that want to know us. You will come through the other side, and as trite as it sounds, you'll be stronger. You CAN do this. Hugs.