CYA of 100 came from use of trichlor or dichlor for chlorination, or maybe from a very large dose of stabilizer added last year. The only way you're going to be able to lower it is to partially drain/refill. The high stabilizer level is also why your chlorine is still in the pool--that's why I intentionally run my CYA high so that I don't have to add chlorine as often.

I'm not sure if I'm reading your post right, but the muriatic acid that you put in is not going to lower your chlorine levels--it will lower your pH. It appears that you're going to need a good bit more, however, I agree with Dave's advice to use the calculator, and get the pH down to between 7.0 and 8.0.

Janet