How to tell my girlfriend that i was cheating?

I have a girlfriend that i love very much we broke up last mouth and i did not do anything special but made it up about a week ago and i recently found out that in a party that was taking place day before we made it up well i found out that i was making out and another girl i know it should not be a problem since we were broken up that time but now the talk is going around and if she founds out she might get really hurt by that or leave me and i don't want to lose her ever again :( what should i do ?