I don't know why this happens, but it does, and don't know what to do about it.

I guess i should tell you a rundown of my story, well here it goes...

when i was in middle school, i met my first boy friend and we were together 5 years. all through those five years he treated me badly, even gone as far as cheating on me since year 2 that we were together. now not realizing at the time, i was very much in-love with him cause he'd been my first with everything. once we'd broken up I thought my life was going to get better, and it did when i met my next boyfriend in my 2nd year in high school. but there was some complications with that I was still focused on my ex. I wanted to forget him, but i couldn't, and my current guy made me choose to not talk about him any more, so i did and never talked about him again, but my feelings were still there.
about 2 years later my Feionsie and i are living in Nashville, Tennessee because of hurricane katrina and we are doing fine, all the way until my ex some how gets in touch with me and invites me to a mardi gras ball. I wanted to go, but i knew that i shouldn't so i did a stupid thing and asked my feionsei if it was ok that i go, you might have guessed his reaction to that, he asked me to leave, he was going to put me on a plane back home. after about 45 minutes of us fighting he regreted the things he said and asked me to stay with him, and of course i did, and we went on with our lives as normal.
in about a 4 year span my ex has repedly chatted with me, and I responded to him every time, with my feionsei finding out every time i have done so. and asked why i did it, but all i could say was "I didn't know why i did it, and i wanted to see what he would do with the information given." but this last time took the cake. my ex wrote me again wanting to get back together after he found out that i was having trouble with my feionsi and talking about things he was going to do once i was with him. I wanted it so much cause i wasn't getting it with my feionsei, and that was clouding my judgement with the love i have with my feionsei. well one of the things he did was ask me if i could send him *naked* pictures of myself. knowing that i would get introuble doing so with my feionsei i did it anyways not really knowing why i did it. after all it was done he found out by my ex telling him that i sent them to him, along with all the things we talked about, and now im in a pickel of weather i should stay with my feionsei or leave. My feionsei says that he is so much in love with me that he doesn't want me to leave, and I him so it is hard on the both of us with him knowing i sent my ex naked pics. so what do you think I should do, and what do you think made me do the things i did?

any help would be much appreciated.