Help? );

My boyfriend can't enter me when we try to do missionary. I can't explain it. It's like my body is too short for him to put my legs straight up in the air. If he tried to enter me his penis would be around my bum. If I try it normally (to wrap my legs around his hips) he can't reach because my hips are too small for his to enter. He's got an average size around 5.5 but this position is one I really want to do but trying to get into this position often decreases his sex drive because he knows I want to do do this position but we can't ever achieve it. Any advice?