You have to be honest with both of them. first, talk to your girlfriend, tell her how you feel, and remind her how you feel about your current relationship, you have responsibilities to her primarily as your girlfriend. If she understands, you have a great girlfriend, respect her wishes, especially if she asks you to cut back on talking to the other girl, which probably isn't helping how you feel. If she understands and trusts you to behave without any change, you have an exceptional girlfriend, and should be punished should you ever betray her. Talk to this other girl and explain how you feel, let her know the situation and I'm sure she'll understand that you already have a girlfriend and need to focus on her.
This could go another way though, and your girlfriend dumps you, in which case you could try with this other girl, but at least have the decency to wait until the bed is cold. I've found in the past that honesty is the best policy, and while it is often hard and can cause blazing rows, the phone call the next day, and the honesty in that, will define your relationship...