I was in a similar boat as you are when we started doing this. We became friends with a truly HOT couple and the issue kind of went out the window.. She was super petite and tiny, her boobs were way perkier than mine could ever dream of being (some girls just have those kinds of boobs ;p ). What has helped me was learning what MY partner liked about me. Not even compared to other girls, but just what made me special to him.
The other thing that will help me is getting myself more in shape and being healthier.
Those are the two things that have helped me