Join Date: May 2009
Location: Mid Northern TN
Assuming that your horse is healthy, your tack fits, and there's not a severe need for some chiropractic work (more likely than you think, and worth getting checked out if you have someone in your area), then...
Keep at it! As a stiff 10 year old, I'll bet she's had years of practicing being stiff that you're going to have to overcome. It will take lots of patience, and lots and lots of practice! I would feel reasonably certain we're talking on the scale of months here- not weeks or days.
So practice, practice, practice both on the ground and in the saddle! I'm still working on getting and keeping my horse soft and flexible- it's a never ending work in progress. :)