Struggles on top

I don't know if I'm the only one.. But I've been sexually active for a few years and I still have a really hard time being on top. It's my favorite position, but I have a hard time with the actual "up & down" motion so my boyfriend has to hold under my butt and do a lot of the work even though I'm on top. Any tips for comfortable way to ride where I can put in more work myself?