Yeah, what people have said - it's all about how you were "potty-trained." You got trained to sit, so your body isn't used to "letting go" when you're not "in position." If you pee in the shower, you've essentially re-trained your body to allow itself to pee when standing, as long as that water is hitting you. So you might want to try in the shower first (which is what I'm doing right now anyway because I dribble everywhere but through the tube, it seems...). Then transition to standing in the shower with the water _off_, so you retrain the body in a safe place but get rid of the stimulus of water hitting you. That will probably help.