Most Helpful Girl
If your plan is to stay with your girlfriend, then I would say don't tell her. Crushes happen, but there's no sense in making her feel insecure about it (unless you've crossed any boundaries with the other girl). Distance yourself from the other girl so that you're not encouraging further feelings or putting yourself into tempting situations.
Most Helpful Guy
What ur suggesting is cheating? If it come to u must be with the other girl break up with the one u have noe