I have made a cowboy hat brim (3 days ago) out of thin fondant and it is NOT drying. I didn't make the fondant myself, it is from Michaels(Wilton), but I made sure I rolled it out pretty thinly.
The outside seems to be dry enough, and you can pick it up without too much damage, but the under side (that was next to the waxed paper) is still a little soft. If I set it down without its form, it starts to drop. I need this to dry NOW!
I can't help you with your immediate problem, but you can use a 50/50 gum paste and fondant mix to make it dry hard for next time.
Maybe try dusting with a cornstarch/icing (powdered) sugar mix?
What are using for your form??? If it is a styrofoam you can remove the wax paper that is between the two so air can get to the underside of the rim. The wax paper is keeping air from getting to it to let it dry from all sides - it will take a lot longer when air is only getting to one side.
Also like emrldsky suggested using 50/50 mix will help it dry faster and harder.
I read on this site before to put it in your oven w/ just the light on. Don't turn the heat on and the heat from the light helps dry it faster. I 've never used this technique before but it might be worth a try.