Is he a true drug addict? You should never have drug addicts as boyfriends in the first place. It is just not worth it. Life is very difficult even without addiction problems. To add addiction on top of everything else is insane. Do not do that. Imagine he overdoses and dies? Do you want to go through that experience?

I do not think that rehab places have a good record of success. Agree with s4ndm4n2006 that you are way too young to compromise in such a way.

You are asking a wrong question and your mind is preoccupied with a wrong topic. The topic you should be thinking about is how to detach yourself emotionally from the bf. I know it is very hard and love is a strong feeling that is hard to overcome, but you need to have some safety rules set for yourself, and the earlier you develop them, the better. People who are abusive in any way or severe drug or alcohol addicts should not be considered. I am talking about true addiction - I am not suggesting you only associate with people who abstain from everything... I am talking about severe addictions.

And, you should not be rescuing anybody and making personal sacrifices for anybody. If you feel the need to rescue people, you should see a T about it to nip the problem in the bud. 21 is a great age to become aware of this problem and deal with it before real damage is done. That you are in love with a relapsing addict and talk about him in such romantic terms makes me immediately suspect that you do have the problem of wanting to rescue people. Let us hope I am wrong but see a professional to be checked out.

Regarding your dilemma:
I don't know what to do. I want to be happy.
But I don't want this to affect my relationship.

You should have your priorities straight. Your first priority is to be happy. Relationship comes second. Go with the guy who appears from your description to be drug-free and nice enough. Let us hope that perhaps something changes and you start liking him and forget about the drug addict. That would be the best case scenario. Unfortunately, it is unlikely, because young and old women alike tend to fall for people who are not good for them and disregard nice people, but you should at least give it a try. If it does not happen, at least you will have had a good time. You say that you are "very anti-social/quiet kind of person" so it is not like you have 500 friends on FB, right? So appreciate the one you have and have a good time. Ultimately, your goal should not be to keep the r/s but do everything possible to sever the r/s because you are courting disaster by associating with a drug addict. You are also romanticizing the situation - "he is several states away, he will be away for idk how long" - there is nothing romantic about it; responsible people who care about their gf's have their priorities straight and do not relapse because they want to be with their gf's. So you should be able to see that there is nothing romantic about your current station in life and try to change it for the better, as soon as possible.