As you've noticed, the problem is that rubbing the tender underside against the dancebelt can actually create an erection. I think most guys suffer from this problem but are too shy and embarrassed to talk about it. It's not something you would be comfortable asking the guy next to you in the dressing room about!
Not only are erections inside dancebelts embarrassing, but they can also hurt if your penis is facing the wrong way. Be sure to arrange yourself when getting dressed to make sure that doesn't happen. Then, try to get your mind off the friction that is exciting those tender nerves, concentrate on your dancing, and hope for the best. Sooner or later you'll get soft again. You might try wearing your dancebelt around the house to help get you used to the feeling of wearing it, so you don't get turned on every time you put one on. Otherwise, all I can suggest is wearing a loose T-Shirt over your tights.
If you are facing a white tights performance next week, make sure to get a flesh colored dancebelt that won't show under the tights. And hope pre-performance jitters keep your mind occupied. Good luck![/i]