Hey n.a.e.,
Sorry to read that you're not feeling well. You have to accept that a lot of things are just not under your control, and that its fine that way. If you could dictate your gf's every move, that wouldn't make you happy either. The worst thing for a relationship is mistrust. When one party does not trust the other fully, then the very foundation of your relationship is rotten and it won't last. Also, thoughts like these can act as a sort of self fulfilling prophecy: you worry about her cheating, she senses your insecurity, this drives her away. Don't be that guy. Let her feel that you trust her. I'm not saying you should let your guard down completely: anyone in a relationship should keep his/her eyes open. But theres a difference between being careful about getting burned and killing the fire just to make sure.
I'd stay away from the caffeïne, if i were you. Stuff is toxic to the mind.
Cheers,
Max