I think what you are feeling is normal. Of course when you see a hot girl, you will get excited. It is a natural instinct. Just because you have a gf, it doesnt mean these natural urges go. I would learn to live with it. Admit to yourself, yes she is hot but what else is behind the surface? Like with the Kardashians, they are hot but they are not that attractive because you know the have nothing in their head haha so yes hot girls are mesmorizing but unless you plan on replacing your girl, I would just accept that hot girls exist and be open to yourself about it. Don´t resist it!