L post

I'm on a flight to Amsterdam for the weekend and right before boarding at the gate I found out my gf is still talking to her ex. She didn't tell me but I caught a glimpse of her iMessage when she opened it in front of me. Not sure how to proceed. I made it clear to her that I didn't want her talking to him about a month ago when she mentioned him and referred to him as "her man" (I know, I know, I shoulda ended it then) but when I got angry at her then she apologized profusely and told me she loved me and would never do anything to hurt me. I guess her performance was very convincing. Anyways she hasn't been any less affectionate since then so I forgot about it. But yea, I'm on a flight to Amsterdam right now with her (shes sitting in another seat) and I don't know how to proceed. My job allows me to get very cheap tickets on all kinds of flights, so she wouldn't have a ticket back without me. I don't know if I should break up with her when we land and just fly back myself. I feel so disrespected and taken advantage of. I'm shaking as a write this. I'm in love with her. This is the first time something like this has happened in my relationships. I'm 26, shes 22.