FileVault Conversion Progress stuck after Yosemite Upgrade

I'm on a 2012 Macbook Pro, 500GB HDD. I enabled FileVault during the install and ever since my computer has been dreadfully slow.

After doing some Internet investigation and doing diskutil cs list in the terminal, I see that FileVault is converting my hard drive, at 83% progress.

Unfortunately, it's been at 83% for over two days now. Sometimes when I check the progress is "Failed," but that's transient - usually it's 83%.

I can't turn off FileVault in System Preferences, and am not nearly Mac-savvy enough to try something else. I'm afraid that I'll have to format and reinstall.

I suspect this is the cause of my computer's slowness. Any suggestions?

diskutil cs list

CoreStorage logical volume groups (1 found)

|

+-- Logical Volume Group CEF82913-95DA-4413-9FF5-25F6F1A4E93E

=========================================================

Name: Macintosh HD

Status: Online

Size: 499248103424 B (499.2 GB)

Free Space: 18972672 B (19.0 MB)

|

+-< Physical Volume 5427F52F-8AFF-4372-9125-545A93D3DA74

| ----------------------------------------------------

| Index: 0

| Disk: disk0s2

| Status: Online

| Size: 499248103424 B (499.2 GB)

|

+-> Logical Volume Family 040D0C49-1406-41C3-B971-03F39AC50596

----------------------------------------------------------

Encryption Status: Unlocked

Encryption Type: AES-XTS

Conversion Status: Converting

Conversion Direction: forward

Has Encrypted Extents: Yes

Fully Secure: No

Passphrase Required: Yes

|

+-> Logical Volume 8F667D6A-3AB9-4A13-A517-21670B2FD1F3

---------------------------------------------------

Disk: disk1

Status: Online

Size (Total): 498876809216 B (498.9 GB)

Conversion Progress: 83%

Revertible: Yes (unlock and decryption required)

LV Name: Macintosh HD

Volume Name: Macintosh HD

Content Hint: Apple_HFS