Mental unfaithfulness?

3 weeks ago my girlfriend of 15 months said she didn't know if she wanted to go out with me anymore, anyway we spoke about it and it turned out she just thought we were in a rut, and we'd try and change things and do new stuff. Even so I've been insecure for the last 3 weeks since then because we'd been ok up until she told me how she felt. Even though we said we'd try and do new things, she's not been affectionate to me or complimented me like she used over the last 3 weeks and she didn't seem as interested in me in a sexual way. In my mind I started thinking about other girls I knew and imagined being with them in a sexual way cos I didn't feel wanted by my girlfriend. The thing is I dont want to be with them, I want to be with my girlfriend a lot and I genuinely loved her, but I was just frustrated and craving affection which I'd had for over a year. I admitted to my gf that I'd thought about people in a sexual way and explained why, and now she's saying she doesn't know if she can trust me and basically making me feel awful for having done this with the whole 'if you loved me you wouldn't think of anyone else..........' but I was just missing and craving affection and the feeling of being wanted which means so much to me. How can I prove to her that I love her make her know that it really is her that I want? And also, for other people in relaitonships, do you ever fantasise about being with people, 'real' or famous? I do sometimes, but it doesn't mean I wanna be with anyone else; I still love my girlfriend and dont want anyone else. Thousands of people in good relationships watch porn dont they? I dont, but thats kind of the same thing ie. fantasising about someone. Thoughts anyone? Cheers very much.