You are not making any mistakes. Your pool has a lot of organics in it and it takes a ton of chlorine to get them out.
You have prolonged the process just a bit by not replenishing your chlorine often enough but that's water under the bridge.
Every chance you get, test and boost that chlorine back up to 20 ppm. It's mandatory.
That your CC's are reducing is a very good sign. The organics are getting burned up..... don't get cold feet now.
Keep your chlorine up. Re-read "How to Shock Your Pool" up in Pool School. It is a lot to take in but you need to be filtering, backwashing, cleaning, vacuuming, brushing and keeping that FC at 20.
Your situation is not at all unusual and your pool will clear soon and your FC demand will reduce to normal