Just because you are attracted to another girl does not mean your current relationship has gone south. If it's just physical, there is nothing to worry about as that's perfectly normal. Of course acting on that attraction would not be the best idea if you wanted to keep your current girlfriend.
However, if it's more emotional, something is missing in your current relationship. You need to figure out what you are not recieving from your current girlfriend (and what she is not getting from you) and decide if it's possible to rectify things. Every relationship you will have will require work. This will never end. There is no such thing as "meant to be", just people who are committed to each other. If you don't think your relationship is worth the effort, you are only prolonging the inevitable.
I suggest you answer these questions:
1) Are you happy in life with your current girlfriend?
2) If not, do you think you can achieve happiness with her?
3) If so, what needs to be done to get there?
Whatever you do, figure this all out yourself. Do not involve either girl or you will end up shooting yourself in the foot.
Really, I'll bet you already know how things will turn out. It's just a matter of getting to the point where you are willing to act on it, either way. I wish you luck in this tricky navigation