I used to have thoughts of what other girls would be like when I was with my ex g/f. I realized that it was not the temporary pleasure that matters, but the permanant, everlasting pleasure.
The feeling of remembering how good of a time you had being with that girl ,not what you did with some other girl.
Relationships are far more valuable than any physical desire can fullfill.
I'm quitting drugs, and that is a desire I have had for 4 years now, and I am 17.
It finally took me 5 times being caught to realize, "Look Tyler, this isn't what you need to be doing in your life and you need to focus on what is important."
How does this relate to you?
You need to not focus on the fun at hand (drugs for me) and focus on the goal in the long run(staying clean for me). Look forward to the long lifetime of love you can have with a girl, and not some physical desire.
Look at the end results, and determine accordingly. If you want to be with this girl, then do your current g/f a favor, and breakup with her. I had to..I did not like a girl like she liked me, and I brokeup with her(it is more complicated though)
What I am saying is that if you think you are going to feel like this again, do your girlfriend a favor and breakup with her now, not hurting her as much as you would a year or two from now, when feelings are much deeper and passionate, and one can be truly hurt.
sorry if my personal stuff got boring, I like to try to relate with people..its a habit, I apologize..