How can i stop being shy?

Iv only slept with 3 guys but all were relationships and were with each for long term time frames and have always been shy in bed. Im quite confident in every other way, in how I present myself and approach guys though. So I dont know why i have such trouble when hooking up. Iv almost always been on the bottom during sex and feel very self-concious when they asked if I wanted to go on top. I also have problems taking initiative and rubbing/grabbing guys when hooking up. I want to seem like im participating and getting in to it. Im also very quiet during sex. Help is definatly needed any tips would be much appreacied. I am 21 by the way.