If you talked to another girl behind your girlfriends back and thens she found out what do u do?

Ok so the story is i talked to this girl behind my girlfriends back to stop some rumors but then i deleted all the conversations and my girlfriend freaked out about it and got all mad at me, and now i feel like **** about it and not a day goes by where im not sorry. So now every time we fight she brings it up and it makes me feel even worse everytime, what do i do?