Mac password not working?

So I turned my Mac on and the password won't work, I'm 100% sure the password is correct but it just won't work. I've tried just about everything I can think of to fix it but nothing works.

I’ve tried the Password Reset Tool

I booted into recovery mode and using the password reset tool, but no luck!

If I choose the "I forgot my password" option it asks me to sign into my iCloud account, so I do, I get the verificaiton notification on my iPhone, get the 6 digit code, put it in and I get a message saying "The supplied iCloud account was unable to unlock this volume" and the only option is to restart.
Or sometime it lets me successfully sign into the iCloud account and gives me the option to type in a new password/hint, so I do and then it says "Reset Password Failed" and "An internal error occurred"

If I choose "My password doesn't work when logging in" then it asks me to "Enter your password to unlock the volume "Macintosh HD" - Here's where I think the problem is, the disk shouldn't be locked! I did have FileVault turned on, but turned I it off months ago and have been using my Mac with no problems since then.
Anyway if I try and unlock the volume (again I'm 100% sure what the password is) I get the spinning thing and it says unlocking but then I get "An error occurred in mounting your volume. Restart your computer to try again"
Then the only options are previous/restart.
But sometimes when I try this option it won't accept the password (yes the exact same password) I just get "Incorrect password"

I’ve tried single user mode

I've read other posts online which suggest trying to boot into single user mode, but if I hold cmd + s on boot it just takes me straight to the login screen... which of course no passwords work!

I’ve tried unlocking/mounting via Disk Utility

If I boot into recovery mode, open Disk Utility and click info for Macintosh HD under File System it says "APFS (Encrypted)" - But again FiltVault should be turned off! I definitely turned it off (and no one else has access to my Mac)

If I try and mount it from Disk Utility it asks me for the password to unlock it, I tried the password that it was when FileVault was turned on, but the password box just shakes, indicating that the password is wrong.

I’ve tried unlocking and mounting via terminal

I've also tried what they suggest here: Unlock or decrypt an encrypted APFS boot drive from the command line | Der Flounder

diskutil apfs unlockVolume /dev/disk2s1

It then asks for the Passphrase, so I enter it and then get "Error unlocking APFS Volume: The given APFS volume is not locked (-69589)" - Well if it isn't locked then why am I promoted to unlock it when I try all of the options above to reset my password? Or mount the volume!

So then I try:

diskutil apfs unlockVolume /dev/disk2s1

and it says… “Volume on disk2s1 failed to mount; it appears to be an APFS Volume which might be locked

try diskutil apfs unlockVolume”

So is it locked or not?

Finally I tried decrypting volume via terminal

I ran diskutil apfs decryptVolume /dev/disk2s1

It asks for passphrase, I enter it and get:

“Starting background decryption of disk2s1 using the Disk crypto user as authentication
Error starting background decryption of APFS Volume: Unable to get the decryption state for the given APFS Volume (-69575)”

Does any have any other ideas? Is the disk corrupt? Is FileVault enabled or not?

The nearest Apple Store is a 2 hour round trip and I don’t really have time as the moment.

I've got the Mid 2015 15" Retina MacBook Pro (512GB SS)

macOS High Sierra 10.13.2

I've got a few different backup options, Time Machine via USB and Time Machine via network share, but I can’t seem to access either of these to restore from (not sure if related to this issue or not)