You made a mistake. You doubted your own judgement and allowed yourself to be pressured into something that didn't feel right. Don't feel bad. It happens. It's one of the perils of youth but you are wise enough to recognize the problem and seek a solution.
So how do you get out? The simple answer...simply tell him that you agreed to try it and it didn't work for you. Thanks but no thanks.
I may reading things into it, but your language leads me to believe that it won't be that easy. You say that it has been bad from the start yet you have been together for 6 months? My gut instinct (which, as a woman, you should learn to trust) tells me that this guy is bad news for you. First, this was a "very close friend" who threatened you with the loss of his affection if you didn't give him more. Did "more" involve sex? If not, what was the point in the change in title? True, lifelong romantic relationships are built on friendship not sex. True friends want what is best for their friend and they want they strive to help their friend feel comfortable. This boy has pushed you into a relationship that you did not want in order to get something that he wanted. You are being used. Second, his threat of "doing something bad to himself" is frightening to me. I'm not frightened for him. I'm frightened for you. This is emotional blackmail. He is attempting to make you responsible for his decisions. That's not fair. You can not make him hurt himself. Only he can make that choice. And if his personality is so unstable that suicide would be his response to an emotional upset or rejection, this is not the type of person that you want to be involved with. Trust me-I've been there.
At 19, my boyfriend was so upset that I wouldn't move in with him that he threatened to kill himself and even drove to the edge of a cliff to prove the point. I relented. Five years later, I escaped what had escalated into an emotional and physically abusive relationship based on his possessiveness of me and my fear of being alone. Don't do it!
I understand that you love him. If you believe that his threats are serious and that he would really hurt himself, tell someone (his family, a teacher, a minister) about your fears before you tell him. If you think that he may be able to talk you out of it, have someone with you when you tell him. Do not let yourself feel responsible for his problems. Nobody can solve another person's emotional difficulties for them. It may do you good to look at why his blackmail was able to work on you-what fears of yours has he touched upon? These are things that you may want to talk with a therapist about but this are your issues and you must deal with them and you must deal with them before you get into a relationship.