That's weird especially when it's comming from the girls side.
But honestly, I don't think there's a need for that much stress. Sexual desire always dies down from what I've experienced and heard from others. I'm sure there are ways to maybe fix the problem, my advice would be to start fucking other girls. It's the number 1 solution to not getting bored with hitting the same old pussy everyday, plus since you ain't really hitting it, you really aren;t losing anything. As a matter of fact it will take your mind of that subject, keep you busy and entertained.
I wouldn't be too sure about her banging another cock, although in a girl it's very possible cause from a guys perspective I don't think anyone with balls atached to their body will be giving a fuck about banging some strange ass by a mistake and going to her girlfriend the next day. Save the guilt feelings for when you are 30 with kids.