HELP!!!

I was in a serious relationship (I’m only 16, but I do consider it a serious relationship) that had a harsh ending when I found out my boyfriend had not been faithful to me, cheating on me with one of my “friends.” Those first few days were really rough. I was mad, but hurt more than anything. We decided we would be cordial and eventually friends, about a week after the breakup. But I soon found out that he wanted me back, and he told me he would fight for me any way he could. He said he wanted to marry me someday, he never wanted to touch any girl besides me ever again. Etc. I friend-zoned him, but we soon ended up getting back together. After dating for a short period of time, we both knew it wasn’t working. I had no trust in him, I wouldn’t let my walls down with him, I always felt jealous of everything, and I was always picking fights with him or bringing up the “other girl.” it wasn’t healthy and he ended the relationship. I didn’t even cry. I knew it was the right thing to do. It was time to move on. That was prom night. And the same night, he slept with some other girl. It hurt to know that he could move on so fast, but I realized that moving on brought endless possibilities for me, too. I went on a few “dates” with this guy, but even though he was perfect and SO hot, I just missed what I used I have with my ex. Once again, my ex came crawling back, saying he was lost without me, that he tried to move on but it didn’t work. He called me crying every night, begging for me to come back. I told him I had moved on and wasn’t coming back. He was in such a bad place. He became clinically depressed, wrote sad poems all the time, was always crying to me on the phone. He even cried at school when he saw me once! (and he is a jock type - so this is all weird for him!) I kept denying him but he told me that since he is going into the army in July, he wants to know I’m his when he leaves an when he comes back. He said he wants to marry me when I graduate. He said he would never be able to move on from me. I spent nights crying my eyes out on the phone with him. I felt so bad. And I think that’s why I recently took him back. Again! I think I felt bad.. But I regret taking him back. I don’t want a pity relationship. But he’s going into the army soon and he had said earlier that he didn’t care if he came back or not. That scares me…. I want to break up with him but if I do, I don’t want to have to deal with this anymore. It kills me inside and it’s gonna hurt him SO much. He’s in love with me, and I love him …. but not nearly as much.. :/ what do I do? My friends and family don’t even know that we are back together. They would all kill me if they knew. So I have no one to support me. Please give me some kind of advice. I’m ready to move on with my life. And I need him to be too. But we have only been back together for less than a week and I just need to be done messing with his feelings!! I can’t handle this stress in the midst if finals!! How do I break up with him, without ruining his life like that again?