Welcome!
I *hope* this is a simple fix.
If you crawl underneath the driver's side of the car with a can of wd-40 and a rag, you'll find a "lever" that sticks out of the side of the transmission, with a cable attached to it.
I believe one mechanic called it a "kick-down cable."
Yours is quite likely covered with grease, dirt, grass, leaves, small animals, or any combination of the above. Clean everything off, and lube it up, it should shift like new.
I have to do this on my '86 Town Car every few years.