<human>: Me and my GF have been togeter for 15 years since 2 weeks ago. She went on a hike with two of our friends, stayed at my cabin for the weekend. Our friends are 1 girl and 1 guy, they are not a couple. She admited being attracted to the girl but i clearly said i was not so cool if i was'nt there with them. At the cabin, they got drunk and the three of them had sex. From what she tells me, she was mostly involved in girl to girl and her and the guy mostly kissed, i assume some grabbing but they said no penetration. We have been togeter for 15 years, she has always been very strict about cheating, that it was something horrible... I cant believe what happened... I feel so lost, so sad. She says she regrets a lot, it was a mistake, she loves me, etc.. I cant shake the image, i see them in my head and i cant get rid of the image.. She betrayed me and i wont be able to thrust her again. I texted the other two to tell them that i was really pissed off, they said they where sorry, that it just "happened"... I understood that they were gonna keep it low and i think it's for the best. Now after 15 years i feel betrayed like i never was, i dont know how to react. I kicked her out but i asked her to come back the next day, we spoke but it's not really helping. I have no one to talk to, im alone and im sad. Im thinking of doing stupid stuff, i need some help.

Can you succinctly summarize the text in 1-3 sentences?
<bot>: Relationship of 15 years, cheated on me says it's a mistake. Feel betrayed don't know what to do, no one to talk about it. Help.