Both genders I need your help! Mainly guys! I really need a guys opinion too!?

I'm engaged with the only guy I have ever and will always love. I can't to marry him. But I don't see why he loves me. We have these arguements and their always about the same things over and over. Not often about once a month now although we used to argue constantly. He starts the arguements but he has great reasons to so I do not blame him what so ever. He found out I would flirt with people on facebook but the thing is I never realized I would. Still now I don't believe I did. Some part of me believes I did but I don't know. I would never say anything out of line. I would just put smiley a lot which I do all the time and I would playfully insult people. But I would do that to everyone. I have stopped and now I just say things straight up and usually no smileys unless I'm speaking about him and our wedding. But it's constantly something we argue about. Another thing is I promise things some times and I would break them. I understand him not trusting me over some things I promise. But I would forget about the promises. I'm a VERY forgetful person and I did not mean to do it intentionally. I have no feelings for anyone else what so ever I know that for a fact. He doesn't seem to believe me though. I told him even before we started dating that I **** up a lot and he would say it's okayy. But now he doesn't let it go. I caught him asking his ex gf who is now his friend about her life. He denies it and says it was a typo. I don't see how that is a typo. And now he rubs all of my mistakes all over my face when I mentioned her (I've known her for a long time since I was 2 and I never liked her what so ever.) and then I remembered that and I told him I did not like her and he went straight to defending her. I don't like that he talks to her because she is a slut and I know it sounds bad to say, but it is true. I don't know what to do. He doesn't trust me, yet he loves me. It doesn't make sense to me. I completely trust him. I just don't trust her I know exactly how she is. I really need some help I don't want to lose him. I know I've really ****** up but I am changing and I have been doing a lot better. He just keeps bringing things up and I'm not the type of person who argues but for the first time ever I did this time and it wasn't good at all.