Nexus 7 2012 is almost unusable after upgrading to the latest Lollipop. What should I do?
The official Google Developers site has the full instructions and image repository on how to install a factory image for any Nexus device, including your "nakasi" Nexus 7 2012. I recommend 4.4.4, but you are free to go back as far as you want until you're comfortable with it's use. 

Cyanogenmod also has a stable 10.2 version (Android 4.3) - you can download the image file here. You can re-use the instructions found on the Google Developers site to install this as well (just target your CM unzip location instead).

BE AWARE: This will wipe your device, and this is all done under your own volition. Flashing a device - regardless of source or instruction - always comes with risk of damaging or rendering the device useless. We are not responsible for it!