FireVault Encryption stuck unencrypting

I used to be able to make regular backups from my MacBook Pro 10.13.6 to a portable drive. Every since I used FireVault to encrypt my mac, it won't back up anymore. It gets stuck mid process. I tried clicking on "Turn off Firevault" to un-encrypt it but it has now been stuck on unencrypting. It says I still have one more day remaining to un-encrypt every time I boot it up. Thankfully, it's still functional. How do I get it to finish un-encrypting so I can re-claim my Mac? I also want to upgrade the OS. I still have High Sierra and it won't let me because it said its un-encrypting.