I agree with Tishie, open relationships only work when both people involved really want to live that way. You are walking a tight rope, but your desire for the other girl sooner or later is going to invade your relationship, even if you never act on it. The best thing that you can do is make a decision of whether you truly what to pursue a relationship with the one you have, which in this case means sacrifice on your part or if you seek something else. As painful as it is you must be honest with both yourself and the girl you are dating.