Are there any good treatments for damaged hair and the correct way to use them?

I have been using hairspray for nearly 20 years. I started using it because it was inconvenient when my hair fell down while I was working, and it became a habit. Now, I use it every single day without fail.

I was naturally born with thick hair, so I didn't really notice it before, but lately I feel like my hair lacks strength, shine, and is stiff.

So, it firmly holds even weak hair in place and makes it look shiny, so I actually find myself spraying even more. I go to the point where I have to wash my hair two or three times beforehand just before going to the salon. Otherwise, the hairdressers don't seem to like it very much...

Still, I always make sure to use a hair treatment when washing my hair in the evening, but I used to just use whatever was given to me as a gift or whatever reasonably priced product I bought at the supermarket.

I apply it to my hair and rinse it off after a few minutes, but I don't really feel the effect of the treatment.

Do you happen to have any treatment recommendations or proper usage instructions? Or is there a better product to replace hairspray?