I've never ed with a partner...Suggestions?

I am 25 years old and I have had plenty of ual partners. I have a good drive and am a pretty open person however I've never actually ed with a partner. I have d with a vibrator, however I've never gotten off with just my fingers. My boyfriend is an amazing lover, however as with every lover I've ever had I've never been able to fully *** around them. Whenever I am stimulated to the point where I may be able to , I pull back, almost as if it feel likes a funny bone or charley horse. I'm wondering if its because I'm embarred or I just don't feel comfortable showing someone that side of me. I'm not quite sure if this is common, or if any can relate, or you may have any suggestions. thank you