If you don't feel masculine around other guys, first you must recognize that your beliefs are causing you to feel this way, and that you can absolutely change yourself.
The best way for you to feel masculine around other guys is for you to feel more masculine around yourself. Try thinking of things you associate with male masculinity, and practice those things frequently.
Maybe you can start working out more or practice getting better at your favorite sport. Whatever activities you associate with masculinity will work. The more you practice doing things you perceive as masculine, the more masculine you will feel.