It really does sound like your dancebelt is too small --- try an XL, if you have the $$ to spend on it.
You could use a full-bottom dancebelt, then you don't have to deal with the evil little G-string.
The other thing you could do is ditch the dancebelt, that's what I did for everything but performances. A full bottom (not thong) men's leotard actually provides a lot of support, and then you don't have a pesky T-shirt falling all over the place either. It works well to wear either tights or men's bikini-style (not thong) underwear under the leotard, and that will ensure that there really is enough support. But get a men's leotard, woman's leotards have the wrong proportions and don't fit as well.[/i]