Hi there!
To decrypt your drive - USE your RECOVERY partition to start decrypting process.
0) BOOT INTO RECOVERY PARTITION (cmd+R when booting) or load into clean system from external drive:
1) open terminal from recovery utilities
2) FOR APFS DRIVE type:
/usr/libexec/apfsd
OR TRY THAT!
sudo /usr/libexec/apfsd
FOR HFS DRIVE:
/usr/libexec/corestoraged
OR TRY THAT!
sudo /usr/libexec/corestoraged
3) TO RESUME/START decrypt/encrypt SERVICE of your drive:
OPEN NEW terminal's window.
4) TYPE:
diskutil apfs list
see your main drive disk0s0 (for example) and write its UUID
5) TYPE:
diskutil apfs listcryptousers /dev/disk0s0
see your username
6) TYPE:
diskutil apfs decryptVolume /dev/disk0s0 -user uuid_goes_here -passphrase 1234567890
Remember!
"1234567890" - is your password of user
"uuid_goes_here" - UUID of your drive
"disk0s0" - your drive partition
7) Check your decrypting status:
diskutil apfs list
P.S. Decryption will be very long. Even if it's an SSD. Most importantly, check that the decryption percentages are slowly but increasing.
Good luck!