I feel the same way. I've been having sex for awhile, but I still hate being on top in the reverse cowboy position. I get leg cramps, it's harder to have an orgasm because i'm concentrating so hard on keeping up with the rythm my husband wants, and the worst is when you go up too high and the penis falls out, then you land on it, kills the mood for the man when that happens thats for sure!
Only advice I can give you is to try it on a couch, or somewhere where he's sitting in an upright position, and try it like that. I never have problems like that and can orgasm very easily too! If your man really wants you to be on top in the "reverse cowboy" position, then you should lay on top of him instead of going up and down. You get the stimulation you need, and it's a lot less pressure on your legs. And if you do want to do the whole up and down thing, switch from doing it face to face, to the opposite direction (trying to explain these things the best I can>.<) hope I helped ya a little!