Overall, I like this a lot. It's a very good description of what it feels like not to give a shit about anything. I didn't get the heroin part, but I like that it's not specific. The only part that was off for me was the beginning. "Spinning" and "Mood-thrashing" are too active and distressed for the speaker's mood. This poem's tone is far too mellow for that kind of opening. I would start with "It looks beautiful on the outside."
The last line has very religious overtones, obviously. I just wanted to let you know that for me, it changed the meaning of the poem into something like: God has brought the speaker to this place of "nothingness," a kind of eternity. On my first read I thought it was about death, to be honest - simply due to that last line.
That being said, I think it's a fine ending. It works perfectly well. But I just wanted to let you know how I read it, in case you are uncomfortable with opening your poem up to that interpretation.
I like the first half of it, but I think it loses it towards the end when you start asking questions. I think the questions detract noticeably from the piece. 'Might it have been' and 'perhaps' bother me. They sound a bit pretentious or something, definitely out of place, anyway.
Questions in poems are great in almost all cases. Really, they are a kind of trick. They bring the reader into the poem and give the illusion that the speaker's monologue is really a dialogue, which allows for more accessibility.
In terms of the phrasing, though, I agree. "Might it have been" is not pretentious, in my opinion, but it is very indirect. In fact, it's one of the indirect terms listed in Strunk & White, if I remember correctly. You could just say: "Was it another opening?" That way you reduce the amount of syllables and words while still retaining the meaning. "Perhaps" didn't bother me, though I can understand your argument against it; it's very high-register for such a colloquial poem.