This is tough, I know. My bladder was so full before my u/s I was actually in pain! The only thing that kept me going was I refused to do anything to screw up the opportunity to see my baby.
The tech ended up giving me a cup and told me to empty my bladder up to the 3/4 line because it was way too full, LOL.
Good luck - once you see your little one on the screen it'll be worth all the discomfort!