I would like to ask you, if you have told him how you feel? Have you told him, in the same way you expressed above, how you are hurt because of his decision to abuse drugs? How you relied on him, and needed him, and put your trust in him, and had it shattered?
Have you told him how you are happy that he is healthy, but the change still doesn't give you back all of the disappointment you gained, when you placed your trust in him and had it broken over and over?
It is possible to love a person, but hate what they do (hate their actions, but love them as a person in general). I believe what you said is rather reasonable, and perhaps your pain should be made known to this person you have loved. I just wonder this, because it really feels like you need to share your grief.
That aside, I understand what it feels like to feel as if someone has died, but they really didn't. To feel like you just utterly lost them... and not even through a nasty breakup, they are literally just a completely different person. All those disappointments.
I can imagine you probably planned some dreams around this person. I can imagine you had hopes, and expectations, all of which were crushed. I can imagine how hard it is to build new dreams now, that don't involve him anymore. I can imagine how hurtful it is to even try and figure out how to re-piece new dreams together now, and how sad it must be whenever you try... you are probably reminded of him each step of the way.
Must be hard to trust yourself again, and others. Must be hard not to feel stupid either, or feel like somehow it's your fault too, for allowing that, or accepting that circumstance... even though he chose it, you must feel awfully abused by both him and yourself for having such a heavy circumstance in the past.
It must be hard to feel like while he is moving up and getting help and moving on, you are stuck here...alone. And maybe you might even feel angry sometimes that he can be happy and just move on seemingly so easily...and you are still stuck in the hurt and pain, and betrayal of it all.
It must feel hard, because you invested so much, and it seems like he really never appreciated you, or needed you like you needed him. You put so much in, and go so little in return, you got a punch to the heart.
He chose the drugs over you, then he chose himself over you...with the therapy. It must feel like you aren't that important.
I can imagine he has crushed many of your self beliefs, by making you question your worth, importance, usefulness etc... many times, because he never took you in, when you were there for him; and instead turned somewhere else, away from you.
But through all of this, I am sure you have also learned that you have a lot to offer, and are amazing, and wonderful... and he is the one who is losing here. To sit here on this forum and still say you have loved him, even with the knowledge of such a heavy fault of his (drug abuse)... is such a very outstanding strength. To be that person who was willing to stick around, in the toughest of times, to try and love this person... your own self-sacrifice.
It is his loss the most.
Maybe he will realize that one day, but I can imagine you probably don't want him in pain either. I hope that you can see, that really what you have written, means that you got through an addiction too...his addiction, for sure. So I hope that you remind yourself that you have the strength to not let it get the better of you, and to still be able to love, and hope....and dream alongside such a sad situation.
I hope that you can have the time you need to grieve properly. I hope that you have support in your own personal life, when you need it, for this grieving. I hope that you will be able to come out of it, and not be left a completely different person either, because you do sound just wonderful.