Riding him

Talking with my boyfriend recently he was mentioning how he has been going really gentle on me and really wants to get rough. I'm totally fine with that except that he wants me to ride him. But here's the problem, I totally suck at it. I don't know why but my hips just don't move how he wants me to. I can go a few times but eventually I get out of rhythm and it just doesn't work. Any tips??