Argh! I hate going on top too. It hurts and I just can't get into it, not like I can in my personal life. I find ways to get around it pretty easily though - seductively licking my lips and commanding to be bent over and ravished works pretty well lol.
When I worked the parlors and a 2 girl, I was always jealous and admiring of how the other ladies could hop on top of the client and make him cum fairly quickly. I'd always give it a good go but mostly it's an epic fail! Unless the client is extremely horny, a regular that I know will love me on top then my heart always sinks when they ask. Just can't get into it.
Maybe it's because I find going on top a very intimate and bonding experience in my personal life.