girlfriend issues

My Girlfriend and I have been together for 3 years. Before we got together I had slept with a large amount of girls and never been in a serious relationship. She wasnt the sort of girl I would have gone for but I got to know her as a friend which led to me falling in love with her.

As with all relationships you have rocky stages. However ours came about a year and a half in. We were arguing a lot and she had gone off sex completely. A few months later she opened up to me and explained that she had cheated on me during the rocky patch. I was obviously devastated but after working things out in my head I decided I loved her to much to just throw things away.

We began getting back to normal and even started having sex regularly again. However I found out shortly after that I had contracted genital warts and that the only explanation was catching it from the guy she cheated with. She refused to have sex with me until it was cleared up and then she put a large amount of weight on, which I didn't mind at first, I still thought she was beautiful. When ever I try to initiate sex she just used to say ''Not tonight'' or ''im so tired''. I started to feel there was more to it than just that so I confronted her.

Her answer was that she didnt feel self concious enough to have sex due to the weight she had put on. I told her I understood and left it at that. I then started to get annoyed when she would happily walk around the house naked and ask me to talk to her whilst in the shower. If I was self concious of my body I certainly wouldn't be doing that. It made me start to question other possibilities.

My head has been going crazy with posibilities, maybe shes cheating again, maybe she still feels weird about the STI, maybe im not very good, maybe she doesnt fancy me, maybe the guy she cheated on was bigger than me and I no longer satisfy her. As you can imagine my head was going crazy with all kinds of thoughts.

I tried to push these aside, but the other day she mentioned that one of her friends cheated on her boyfriend and they broke up and said they obviously arent as strong as us. This annoyed me so much as it wasnt our relatonship that was strong, it was me. I hadnt thought about the cheating in a while until she mentioned it.

This messed with my head so much it led me do something I never thought i would do. I checked her facebook. I hate myself for doing it, but I found out that at least 3 times a week for the past month, she has been searching the boy that she slept with as well as her ex boyfriend. As you can imagine this made me feel much worse and question everything. The part that hurts the most is, that if she is still thinking about the guy she cheated on, is she thinking about the sex they had?

I am so confused and hurt but still madly in love. I have no idea what to think or what to do?

Hopefully somebody can give me some sensible options.

Thanks.