HI Redrose,
The Canadian system is probably different, but when I had to apply for disability, I had to appeal twice before they accepted my application. It was brutal. What I learned was to never, ever tell them I was better (in any way, shape or form!) I had to make my case as pathetic as possible (which wasn't hard. I was a wreck).
I agree with Tink. Anything you have that will make your case, send it to them. I think that letter you mentioned would be ideal.
Good luck. I know it's hard. I'll pray for you!
Ginny