My girlfriend won't have sex with me

I'm a 24-year-old guy and I currently live together with my 22-year-old girlfriend. We moved in together this summer and before that we dated for almost exactly a year. During the first quarter we had sex several times a week, and since then it's been dropping gradually. I can also point out that I am the one that have been doing all the initiation and that itself lowers my self-esteem. After we moved in together this summer it's almost like she fulfilled her goal and now doesn't 'have to' have sex anymore. We've had sex three times in the last 4-5 months. Usually, she just ignores my efforts or pushes me away or it's too late and she's tired. On two occasions I have brought this up for serious discussion, first time was last spring when I said that I wanted to know what's causing it or what I was doing wrong, it only made her irritated. The other time, after daily attempts for over a month I one night simply got so sad and disappointed that I went to sleep on the guest bed. The next morning, she came in all angry and wondered what was wrong with me, I told her that her behavior is completely devastating my self-esteem and that I cannot understand why she simply doesn't want to have sex even though I know she loves it (you're probably thinking she's faking it or simply lying, but I assure you, she really does love it). Again, she was only irritated and angry that I brought this up. I am seriously on the verge of losing it, to bring an idea of how this is making me feel and how desperate I have become I should mention that I have been cheated on once in a prior relationship and that completely destroyed me that time. Recently I very nearly ended up doing the same thing myself even though I am probably one of the world’s biggest anti-cheating persons there are. The prospects of sex not only HOWEVER, this relationship is in all other aspects completely wonderful, and that's what's keeping me from breaking it off. I realize I have a self-esteem issue, and that really doesn't help when you're the one that has initiate, or try to initiate, every single time. Please help me by giving me some sort of advice on what I should do.