Your girlfriend sounds like a cheating whore no offence.
I had a girlfriend once (years back) who said she had no interest in my friend who she constantly flirted with/he wanted her and after we finally broke up after I couldn't handle the flirting/being ignored stuff any more she fucked him that afternoon.
Now I bet you're wondering how this actually helps your problem, well never fear because now that someone has told you the truth, not what you wanted to hear. You need to start coming to grips with the fact that you two WILL break up and she WILL fuck him afterwards. What YOU need to do is start planning a way to break up with her in the most elaborate, painful and disgusting manner possible so that she will forever carry that emotional scar.
You want to be the "dickhead ex" she tells her future boy interests in. Trust me. You WANT to be that guy.
exactly this, im always this guy, back when I was 18 and split with my very first proper girlfriend I destroyed her confidence
First I made sure I already had a girl lined up then I split with her... She got pissed off kept ringing constantly so I answered one time and let her listen to me fucking my new gf
She still wouldn't leave me alone so I picture messaged a picture of her masturbating and a picture of her sucking me off to her mam, dad and brother