Can you post a set of test results?
CYA at 90 is very difficult to manage, and you'll need to maintain a minimum FC level of 7ppm or higher (12 is your target high) to have any effective chlorine in your pool.
40-50 is the recommended CYA level, you could perhaps get away with a little more, up to maybe 60ppm if you're in full sun all day.
So, to lower your CYA level, you'll need to drain roughly 40% of your water. Pool Calculator will help you determine how much to drain.
All this is in Pool School, so do take a few read thorughs there. Additionally, shocking your pool is not a one time event, nor is it a product. Shocking is a process, that requires maintaining shock level throughout the entire process. To do that, you'll need a FAS-DPD test kit that can test higher than 5ppm, and also test for Total Chlorine.
If you've got any testing ability right now, go ahead and provide us what you can. And then have a drink of something soothing and start reading Pool School. It'll make what we say far more understandable.
Welcome btw :~}