how to turn off FileVault when the progressive bar is frozen?

Sorry for responding so late, I've been pretty busy.

Well, your Mac is pretty messed up but easily fixable, follow these instructions:

How to erase a disk for Mac - Apple Support

Afterwards, follow this:

How to reinstall macOS from macOS Recovery - Apple Support

Please keep in mind that erasing your Mac will destroy *all* data, files, games, applications, etc. Though since you say you copied the files you want to some external storage (flash drive, external hard drive?) already I assume you are ok with that.

Sometimes encrypting can take hours or days or longer (I've seen upwards of a week on some posts here). The speed all depends on various factors such as the type of drive (hard drive vs solid state) for example. As I said, you cannot turn it off in system preferences, it must finish to "turn off."

How long has it been encrypting for?

Simple answer - you can't. You have to let it finish before you can turn it off, at which point it must decrypt everything.

Note: FileVault won't encrypt when the computer is not plugged in to power (if it is a laptop).