You might have a corrupt download of the update. (not anything on your part but on MS servers)
I found directions to remove everything in the C:\Windows\SoftwareDistribution\Download folder
I'm pretty conservative so I'd hesitate to remove everything. Have a look in C:\Windows\SoftwareDistribution\Download
if you see some entries from the past two weeks (the time period you've been trying to install the update), I'd select just those for removal. Or you could move them out of that folder and onto your desktop to archive them just in case removing them doesn't fix the problem. Then you'd have them to move back into the folder.
After you either removed or simply moved the suspected problem downloads out of the folder, reboot. Try the update again.