On my Mac I cannot restart without agreeing to update,. It downloads, installs, says restarting and then just stops. How do I get out of this loop?

Corruption could have occurred in the Application or in an associated preference or support file.

The best remedy is to totally uninstall TurboTax and all associate files and then re-install. That will not delete or change any .tax2018 data file that had already been created with the tax data.

To uninstall the application and all associated files need to be deleted and then the application reinstalled. If there was a state then the state must also be reinstalled.

Here is a FAQ for how to uninstall Mac
https://ttlc.intuit.com/questions/1901210-remove-uninstall-turbotax-for-mac

https://ttlc.intuit.com/questions/1901209-reinstall-turbotax-mac