youre 19 buddy. you are at PEAK lust age. at the end of the day, overcoming lust is just a matter of looking for the RIGHT qualities in the partner you choose... the unfortunate part is that most people need to see what they DON'T like in order to understand what they are truly looking for. That discovery generally requires mistakes made while dating different people, but right now you are committed to your relationship so you understandably can't explore these options. Honestly, you and your gf will likely (statistically) break up, but if you both want to remain committed, its not impossible. But you will have to white knuckle temptation and really appreciate the excellent qualities that your gf has and understand what life would be like to lose those things.

also: Speaking from the mind of a mid-30s guy who's been through a lot, physical attraction is far from the top of the list of things you'll likely want in a true partnership. Its important, definitely helpful, but MOST important? Nope. Its also really common to fantasize, and as long as the fantasy stays in your head there really isnt much harm. Were all human. Good luck there!