Girlfriend wanted a break, I am torn inside.

We are in a long distance relationship and have been dating for 5 years. She wanted to go on a "break" just for curiosity and we made the rules very clear on what the boundaries would be. She has been seeing another guy who she was friends with before but they have not done any funny business, just hang out, barely go on dates and I trust her completely, but I don't trust him. I love her more than words could ever describe and we still talk everyday and tell each other we love each other and continue our long distance relationship essentially during this "break". Do I have anything to be worried about or will she eventually come running back to me, ready to commit for the long haul and relinquish any regrets or doubts that may have been? Or is there a chance she may fall for this guy who is essentially a friend/fling? Anyone have any similar situations they have experienced?

Nobody can predict what she might do but there has to be a reason for why she thinks she needs a "break" from you. That's where the answer to your question lies. Is she bored? Does she have someone else in her sights? Is she trying to "let you down easy?" When the love of your life decides she wants to date other people, that's definitely not a good sign.