Maybe he chooses to opt out of facing your pain or of trying to give you any decent explanation. Maybe he doesn't have any decent explanation. Maybe when he's on drugs is the only time he believes he needs a close relationship, and after rehab he's not or believes he's not the kind of person for a close relationship, maybe he believes being in relationship was keeping him in his drug problem?? Maybe he's in denial of some kind, or flipped into a dissociative or other such state and so just blocks you out cos his former life with you doesn't gel with his current view of himself.
Either way, especially by how he's treating you now, you can see he's not fit for you, not capable of respecting love, not worthy of the love you gave him and are trying to still give him, nor even of respect ...
I'm so so sorry for the nasty way you were treated and the hole that left in you. It will heal, especially as you do let yourself accept that you suffered a shocking loss that won't be undone. You loved in good faith; now you need to find ways to be loving to yourself and let your heart grieve and be gradually restored.