You've got to be f*ckin kidding me here. I've seen this one before: girl thinks she spots BBD, takes a "break" from her boyfriend to test her luck, ploy fails (either BBD rejects her, or turns out NOT to be all that bigger/better), girl goes running back to boyfriend with tail between legs.
Okay, here's the deal... Most guys will happily accept the prodigal b1tch back into their lives and try to forget the incident as if it never happened. I used to be one of those guys. I let an ex do this to me once, except it took her a few weeks to come back after she realized the other dude was a DUD. Guess what happened? She did it again not a month later, and I was sh1t out of luck with my balls in my hand for the SECOND TIME. As for you, make no mistake: there absolutely, positively, WAS another guy in the picture. She basically placed him above you, at least temporarily, and high enough above you to try to make the jump (remember the monkey analogy?) Now, how does that make you feel? Bottom line: you are now LOW on her ladder... at least lower than you used to be. If you take her back you will be making a huge error, because not only will you be allowing her to disrespect you, but you will also be disrespecting yourself, which in everyone's eyes is a much more egregious transgression. You will only continue to drop lower and lower on her ladder, until you are at the very bottom wiping the dirt off your lips.
Please learn from my mistake, so I won't feel as if all my days as an AFC were a total waste. Tell her this, "You know, even though I was opposed to it at first, I really feel like this 'break' was a good idea after all. I've had a chance to get out there and do my own thing and meet some new people, and I'm enjoying it. I'd like to continue the 'break' for now... maybe I'll give you a call in a few weeks or something." Then, never call again. When you talk to her, try to avoid saying things like "you blew it" or "too late now" or whatever, you'll just come off as bitter. Talk as if this break was the best possible thing for you, and let her INFER that she in fact blew it.