Girlfriend kissed another guy and now she is guilty?

Guyz help me out here .. My girlfriend the girl i love and always will recently started to push me away from her .. and started to bring out little things to breakup .. i asked her do u not wana be with me because u r acting weird she said she loves me and she wants to be with me .. but she cant be with me anymore because something happened she told me one day wen we had an argument i was wiv one of my guy frenz tryin to explain him how i love u n wat happened n how to mend things .. she said that guy starid tellin me that don u wowi ur man wud be Fu***** about wiv ova girls now and stuff and he kissed me i kissed him back too but after that i realised my mistake and i feel so guilty about it day in day out .. she is tellin me she will neva eva do it again but she tellin me that she doesnt deserve me cos she is done rong to me while i was honest .. and she is very guilty about what happened and it was a one off! she has stopped bein mates with that guy too completely no contact help me