Jaysus, Robin - A few things,mate...
You're only dating her a few months, and you're going on like this already? How old are you mate? And, let's be honest, if this is the 2nd time you've done this in a week, then it's not a mistake, it's something that you do...
I'm not judging you - I used to be a serial cheater, because I was devastated early on in my dating career, and then AGAIN the first time I decided to really try again a few years later, so I did the whole "I'll not let myself get hurt again" thing... Then, after many years of enjoying being single, and never wanting anything serious, I met Amy. And I've never considered another woman since...
Bottom line - Depending on how old you are, you may be being ridiculously hard on yourself... You're not a complete tosser, but you're not a good partner to this yong lady either, right now. Decide if you want to stop f#$!ing around or not. If you do, then do. If you don't, then break up with her, and enjoy being single.
You know what you want - Have the balls to be honest with yourself.
But regardless of what you decide, if the only thing that telling her would accomplish is relieving younof your guilt, then don't do it. Why should she be hurt for your sins?