I am in a stress/deprrssion. Regret?

Hello,i have an issue with my colleague.Basically my colleague is in a serious relationship with her bf.But we got physical for a very very very short period of time.but i got very serious and 4days later she accidently taken my phone and red all 3months old chats where i was flirting with other girls(during those days i was not in touch with my colleague)and from that days she has changed alot.she is not at all the way she use to be.and from the day she red all my old chats,i am in a big regret thinking about what if i would have not flirt with others and all i hate that bad days.Its hurting me like hell.she use to be chat with me and call me everytime and everyday and i have lost those opportunity just because of that mistake i was flirting with other girls and that to when i was not much in a contact with my colleague .and almost for 1.5months i told her that i am serious abt her.it'll be my biggest regret if i lose her bt no chances given to me.how to handle this regret?please help