I've cheated on my girlfriend for the second time, not sure what I should do about it..

So, my girlfriend and I have been together for about a year or so. I cheated on her about two months ago and, feeling awful, told her about it. She cried and dumped me, but recently we've been on the verge of getting back together. Last night I was at a party, got drunk, and hooked up(no sex) with a girl that has wanted me for a long, long time. She and the other girl that I cheated on my girlfriend with were the only two girls I've ever given in to, and there have been some others. My ex-girlfriend is probably the nicest, most relaxed, and best all around girl that I know. When we hang out, I always have an amazing time. We laugh constantly, love each others' company, and just frankly can't get enough of one another, despite the problems we've had. So, what I want to know is, if it would NEVER, EVER happen again, should I tell her about it?

Additionally, the girls that I cheated with were both close friends of mine. Neither is emotionally attached to me, and neither would ever say anything about it. I can trust them on this, seeing as they know enough about me to ruin my life and haven't as of yet. So please reddit, help me out here... Do I tell her and likely end it for good, and probably hate myself/be depressed, or do I keep it a secret and tell NO ONE, and continue with getting back together with her?