Girlfriend kissed another guy.. need help :(

Well dunno how exactly to write this but here goes.. Girlfriend of 2 months just admitted to kissing another guy. It was about a month ago during Easter break when she went back home (shes European, so she was out of the country). Apparently last year her and her friends used to do aerobics or something similar, but at the same time they have dumbbells in the corner of the room, and a really hot guy used them and her and her friends were all like "ooh hes so hot". He's hung out with her friend group for a while but this break, after only one week (she was only away for two) he asked her out for coffee and well yeah.. they kissed. I dunno it just hurts a lot.. mainly because she actually agreed to go out with this guy, and also I asked her and she admitted to kissing him more than once.. like for the whole hour they were together.. and it was a full on french kiss or whatever with tongues and stuff. And then to add to it, shes been cheated on twice, once by her ex that she was with for two years, and again by her friend with benefits she saw just before me. You would just think she would know how it feels :( Plus whenever we talk about the summer (she will go back to her country for 3 months) shes always saying stuff, like making me promise there will never be another girl etc. I could go on but I think you get the idea.. I feel disappointing, upset, etc.. I dont know. I can tell shes really sorry (she cried for two hours after telling me, and it was the worst thing I ever had to sit and listen to). Shes said sorry ever since. I really want to stay with her and I think she can have a second chance but still.. this isnt something you forget easily. Im worried that if this happened after only one week (out of two) then what will happen in three months? Any advice? Opinions? :(

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