Is this normal? why i feel this way?

Ok, lets start, Im a 21 boy , I have a girlfriend, we´ve been together for almost 2 years.

Before being boyfriends we were best friends and she used to tell me about her experiences with other boys and things like that, she has been (sexually) with 11 guys (not inclunding me) I would love to say they were boyfriends but some of them were not even her friends, this makes me feel weird and sad, I think it is a very high number for her age (20), oh and before you say "yeah , but how many girls you have been with?" I´ve been only with her, she is my first everything, first kiss, first girlfriend, first sex partner. I dont know what to do, we have sex like one time a month, I dont really feel the need of doing it, I could live without it,When she says "lets have sex" I tell her things like "Why dont we go to the cinema?" or "Why we dont go for an icecream?" I think there are more important things than sex, but if we dont do it she gets sad and she start saying that I dont like her, that she is fat and things like that, so I say yes, when we have sex I just think about she doing it with the other guys, I "imagine" things like "did she told that to the other guy0" "Im really the best?" that kind of things and it feels very veeery sad and uncomfortable, I also dont like receiving oral sex, I think it is gross and degrading, im ok doing it but I dont like receiving it and whenever I say "no" to oral sex she gets sad, this is really weird for me, I dont know if im the only one that have this problems , also she likes to send me nude pictures of herself but I dont like it, I think she is degrading herself, so I asked her to stop it. She accepted it, but one time I saw on her phone (I wasnt even stalking, just saw it) that she was sending the nudes to a lesbian , I didnt said anything but it hurted really bad. I think I have to say this , she cheated with some of her ex boyfriends so this makes me suspect she will do it to me, she says she wont do it because im her "soul mate", that im different and unique... But I dont think that she wont cheat on me. So, what can I do? Im sad almost all times, she is a wonderful girl , im not judging her, I feel like maybe im her angel and that I can take care of her, but Im sad each night when I think about this, I see her and I imagine her with other guys, Im asking for your help. Thanks

Sorry if I offended anyone, it wasnt my intention. Have a bright day!