It's a lesson in compassion I guess.
Don't beat yourself up.
I used to get so mad at my bros when they would relapse.
But who was I to judge- when I had enough struggle trying to change myself?
Your BF is still in a relationship with a substance that he used to love, that used to never let him down, that he could rely on....but it probably hasn't kept that promise for him in a long time - yet still he returns to it again and again....he has not had enough, nor committed himself to giving it up - he's still wrapped up in his love/hate for the substance.
You had a great 6 weeks with someone you loved - even though you know how it will end since he hasn't found sobriety/recovery. From all you've been through you had an inkling going into these 6 weeks that what you were hoping would happen was a fantasy. All you can do is stand on the edge, give yourself a little perspective and make a choice about what you want your life to be like. When you've had enough of him/this you'll hopefully still have enough of yourself left to move on fully and fruitfully.
peace-
b