Why do i feel like this

I keep having foughts in my head a lot of my girlfriend havin sex withsome it’s a fantasy of mine now I eithen keep asking her with we could have a 3 sum or just for me to watch her . She keeps saying no all time but we have been doing roleplay in bed were she thinks I’m someone else and she call me a different name it really turns me on knowing that she thinking someone else is havin sex with her . I have it on my mind all time about someone else inside her I hope she doesn’t think bad of me I want her to say she will have sex with someone I eithen try showing her bits to people I do get horny with her I want people to see I just don’t know why I think all this stuff