You certainly do have a lot on your mind. Let’s start with the boyfriend. Of course you are best friends! You two have grown up together and seen each other through some hard times with your brother (and maybe other things besides). But a choice you made for a partner at age 17 might not be the same choice you would make at 25. There’s nothing wrong with that. There is something wrong with a relationship that is so out of balance, with you working so hard to “prove” yourself while he holds back. He may have issues with love and trust but you can’t work those out for him. Hopefully, he will do as you did and find himself a therapist. His past doesn’t have to dictate his present or his future but it will take some work. I’m glad you were wise enough to cancel the wedding. As you describe the situation, a marriage with this young man would diminish your sense of self and leave you always feeling like something important was missing.
My suggestion is that you take a big break, meet other people, and see what develops over the next year or so. You may find someone who loves you as wholeheartedly as you deserve. Then again, your boyfriend just may do his work, grow up , and be a suitable partner.
I am very, very sorry that your younger brother has let drugs take over his life. It must be very painful for your entire family. He is still young. He may yet decide that he wants more out of life. Sadly, there is nothing you can do to make him do that. All you can do is love him and let him know that you hope for more for him. In the meantime, I suggest that you get in contact with the local chapter of Al-Anon/Alateen. The web site is http://www.al-anon.alateen.org. That organization offers practical help and support to families of people who have become addicted to alcohol or drugs.
I’m glad you have some good friends who stand by you. I hope you get back into the social world and enjoy being young. You have plenty of time to find the right guy.
I wish you well.