It's normal for everyone to be self conscious. You sound fine. And if your girlfriend says so then she probably means it. My boyfriend has similar height/weight and says the same thing all the time, that he feels like he's fat and has gained weight, etc. I always tell him how sexy and handsome he is to me because I really mean it.
5 10 180 isn't bad at all m8 girls ussually like guys that are bigger rather than skinnier anyways, if you're unhappy with your build tho you can always go to the gym and work on it. You'll find that once you start you'll get hooked and get your body to a point where you want it to be at in no time.