Being On Top In Bed & Lingerie, Submisive Now Domme?

my bf is great. we get along perfectly and since im bi its great we can talk about others girls and what not it makes life interesting. he never forces it but he likes me on top everyone once and a while...i know he can tell it makes me nervous. and hes great at telling me its fine and keep going..but i think he would feel better if i actually really enjoyed it.alos he wants me to try on lingerie we ave looked at it a few times and everytime he finds sometin he likes i go no way it wont fit or it will look gross...and im tryin but its hard.im use to being submisive in bed...So Is He...so we switch up the Dom Role...but when im in that role i feel really stupid so what should i do here