My LTR broke up with me on Saturday. It's about the fourth time she's done so in the last six months but it sounded like this time was final. I had sex with an old fling the same night. Now she wants to get back together if I make some changes. Should I tell her what happened or feel guilty? I really thought it was over for real this time and so I didn't consider it cheating.

Please no comments on how I shouldn't take her back. She's grown disinterested in me because I became complacent and an asshole (and due to temporary long distance).