Somebody mentioned in another thread that classical musicians tend to have trouble improvising, because they spend all their time and energies learning to play what's on the written page. An over-generalization, to be sure, but it was a fair point. I also think that this dynamic may hold true to some extent with memorizing. If you're accustomed to using sheet music, you may not develop your memorization skills as much as someone else who HAS to rely on memory.

I realize this doesn't give you anything concrete to work with; sorry, just musing. I think the advice that a couple of other folks gave is good -- practice, practice, practice, and try to wean yourself off the written page in stages as you go. Beyond that, I don't have much advice to offer; good luck.