Practice!
If you can pee in the shower, try peeing in the shower while touching yourself there. Once you can do that, try peeing in the tub without the shower. And seriously, there is that fear of peeing your pants. Just do it. Yes, I said it. Stand in the tub and pee your pants. It's not the end of the world. Once you do it once or twice, you find it's not a horrific thing. Then move on to an STP. You can try practicing at home with a medicine spoon, which is really REALLY cheap, and when you master that, then order the STP. That way you're not out any money while you get the hang of an STP.
Jay