I'm confused

My situation is long and complicated, so here goes...

Okay.. So there's this guy that I met online, and have been with for over a year now altogether ( I say altogether because we've broken up a few times, but always ended up back together). Throughout the relationship, I have trusted him and have always been faithful to him. I think our relationship has been strong and it looks to me like it could actually be going places. During our last breakup, we decided that we were in fact were going to get back together, but that we needed some time apart. We decided that a month would be a sufficient enough amount of time. He's always had a short temper and even the smallest thing would tick him off. Well, during our 'break' he got angry with me over something very little, and decided to get back together with his ex girlfriend to tick me off. I of course was furious at this, ( I believe I have all the right to be, considering that I love him and that he only was doing such a horrible thing, to hurt me). I then made him decide on whether he wanted to be with me, or stay with her. I told him that if he were to choose her, I would be out of his life forever and would move on. So he chose me, and then promised that he would never make another mistake like that one again. Also that he would sever ties with her and never speak to her. Our relationship became stronger with time, and then we got some news that his parents had enrolled him into a school far away from where he lives, in the fall. a few days before he was supposed to leave, his ex girlfriend left a comment on his profile on a site that he's on, and it said something like "Be safe! And I’ll keep in mind what we spoke about". My heart dropped into my stomach because my boyfriend had told me that he had her on his ignore list... I then confronted him about this comment and asked what she was talking about. He told me he had no idea. I had a gut feeling that this wasn't true, but I wanted to believe what he was telling me, so I did... The day came that he was to leave. A couple days later, his ex sent me a message telling me that he wants to be with her and things of that nature. Of course, because I trusted him so much, I told her that what she was saying was wishful thinking on her part. she then offered to show me proof... of course I said "alright, let me see this forgery; it'll be entertaining." she then showed me an IM conversation that the two had (while my boyfriend had been telling me that he had his exes blocked} that spoke of how I was the reason why they weren’t still together, and that he still loves her. and some emails they exchanged (while my boyfriend had been telling me that he doesn't, and wouldn't ever speak to his ex again)that had him speaking about if he and I were to drift apart, that he would be with her again; basically reassuring her that they will once again be together. I recognized the way things had been spelled and written, were eerily identical to the way my boyfriend would write them.. I was devastated and cried. I was very hurt and felt betrayal on a level that I had never felt before. I was also very angry, but more hurt than angry. I decided to try to contact my boyfriend so I could confront him about all of this, hoping to GOD that this was all a lie. once I got a hold of him, I asked him about it, and he was lost for words, and didn't know what to say, because he knew he was wrong. I told him to at LEAST admit his wrong doings and he did... it made me cry to hear him admit it. Here: the man who had always been active in making sure that I had always been faithful to him and truthful about my whereabouts, acquaintances, etc., admitting to have betrayed my trust. He apologized for it and said it would never happen again. I then pointed out that that's what he had said before and yet it turned out to be a lie. He was quiet, and then tried to assure me again, by saying that I was the only one he wanted to be with. I believed it this time, but only slightly. Ever since he had betrayed my trust I have been skeptical of EVERYTHING that he says to me. Every day that I had been on the phone with him I had told him that I had been worried about losing him and about the future of our relationship because it's the second time that he'd betrayed me(the first time was when he went out with her during out break). He then got angry and accused ME of trying to break up the relationship, when in fact I'm only concerned, with all the right to be. I then assured him that I wasn't trying to break the relationship up and that I'm just concerned about it. I also warned him that if he were to ever do anything of that nature again, that I would leave him and move on, because I don't deserve that kind of treatment from any guy. He then said alright, and that it wouldn't be a problem at all because he doesn't want to be with anyone and wants to spend the rest of his life with me. He then offered that I come up with something that he could do to prove to me that he only wanted to be with me. I rejected this, because I figured that if he really wanted to, he could easily hide anything from me considering that we cannot see each other. I tried to move past everything that had happened and began just talking about normal things that we would have talked about before this whole situation began. A few days later, he randomly asked me to marry him. I eagerly said yes, because I do in fact love him; I’ve never felt this way about anyone, or connected with anyone the way I do with him. Another thing is that when I get into a relationship, I’m usually able to tell if it is going anywhere, and this is the only one that I actually feel confident about. But just wish that he would stop doing these hurtful things. But now I’m rethinking the reason why he asked me.. Is it because he does in fact love me that much? (because we have spoken about marriage before, he just never proposed), or because he's afraid of losing me to someone else when it could have all been prevented? Please help me figure this out.