Mail app damaged after Yosemite upgrade

Question

I upgraded to Yosemite and among other problems, my mail app does not work. When I open the application it prompts me to begin the message import process. I click 'continue' and the message import begins. The process progresses until it reaches the migration of data, at which point it stops and an error message appears.
>>"The upgrade failed. An error occurred during the upgrade. Click Continue to launch the Mail Import Assistant, which will help you build a new library and import your existing messages." >>
However, when I click continue another error message appears. >>"Your Mail index has been damaged. To repair, quit Mail. Mail will repair the index the next time you open Mail. Your mailboxes and messages will be preserved.">>
When I reopen Mail, this entire process is repeated and I get no where.
I've tried chatting with apple support and it was not helpful. I created an admin account under safe mode and mail opened fine, prompting me to set up a mailbox.  I have cleared my Caches. I repaired HD in recovery mode, disk repair and repair permissions.  Nothing as worked and my mac is running insanely slow.
Someone please help me!

Answer 1

Reinstall Lion, Mountain Lion, Mavericks or Yosemite without erasing drive
Boot to the Recovery HD:
Restart the computer and after the chime press and hold down the COMMAND and R keys until the menu screen appears. Alternatively, restart the computer and after the chime press and hold down the OPTION key until the boot manager screen appears. Select the Recovery HD and click on the downward pointing arrow button.
Repair
When the recovery menu appears select Disk Utility. After DU loads select your hard drive entry (mfgr.'s ID and drive size) from the the left side list.  In the DU status area you will see an entry for the S.M.A.R.T. status of the hard drive.  If it does not say "Verified" then the hard drive is failing or failed. (SMART status is not reported on external Firewire or USB drives.) If the drive is "Verified" then select your OS X volume from the list on the left (sub-entry below the drive entry,) click on the First Aid tab, then click on the Repair Disk button. If DU reports any errors that have been fixed, then re-run Repair Disk until no errors are reported. If no errors are reported then click on the Repair Permissions button. When the process is completed, then quit DU and return to the main menu.
Reinstall Lion, Mountain Lion, Mavericks, or Yosemite
OS X Yosemite- Reinstall OS X
OS X Mavericks- Reinstall OS X
OS X Mountain Lion- Reinstall OS X
OS X Lion- Reinstall Mac OS X
     Note: You will need an active Internet connection. I suggest using Ethernet
                 if possible because it is three times faster than wireless.