I just messed up

I'm a 19 year old guy in college and my (19) GF from HS is studying for a year in Tibet. She doesn't have much cell service or wifi there and as a result we haven't really been able to talk much.

I work at a fast food restaurant 25 hours a week to help pay for college, and today I met this new girl who's also 19. The manager instructed me to train her and she was hitting on me a lot and kind of noticeably but I didn't really do anything about it. We got off at the same time and she told me she wanted to show me something in the storage room and I went and she kissed me and I didn't really pull away and then she started to grab my crotch. I said wait and stuttered a lot but I should have done something more. I was getting into it and she ended up giving me head. I didn't start any of it but obviously I didn't stop and I've felt awful since. My GF and I have been dating for 2.5 years and I've never even flirted with another girl but obviously now I'm a big cheater. I cried for the first time in like 8 years and am lying in bed and can't sleep.

I definitely have to tell her because it's eating me alive but I don't want to ruin her visit in Tibet and she's there until September. I know she likes to hold onto things and I really think this would really diminish her trip. I have been scrapping and saving to go visit her this summer so I could tell her then but I don't want to waste nearly 3k in flight tickets just to tell her. If I snap her or dm her she'll probably get it but it would ruin her trip and she's really great so I don't want to do that.

My mom's in prison and I'm going to go talk to her tomorrow but can someone please offer me some advice. I really feel like I just ruined my life because my GF is honestly incredible and I just blatantly cheated on her