How to deal with feelings of betrayal.?

well i was wondering how i could feel more calm about this situation. Well me and my friend hung out with a couple of girls on a saturday night well one girl really liked me and we had been talking about a week prior to that day and i started to really like her well we drank a little bit and i went inside to go to sleep cause they said they were leaving. Well i learned in the morning that my friend (best friend) had with my date and his date (who were both intoxicated). Well i felt really jealouse and angry well now im in a semi-serious relathionship with the girl that was my date and we are falling in love but sometimes i feel so angry and depressed cause the thought of that night pops into my head and makes me feel extremely bad. I really am falling in love with this person but how can i deal with my feelings? she told me it was a mistake and it wasnt right... and i told her not to worry about it but how can i deal with it?