Delete Yosemite? Find Mountain Lion? Post-Stroke Explanations, Please
Sorry to hear about your stroke. To answer your question, downgrading operating system installs are pretty tricky. This is further complicated by Apple's switch to exclusively distribute OS X via the App Store - if you don't already have access to the Mavericks installer, you're out of luck given that you can't even get the installer on the App Store anymore.

If you do have access to the Mavericks installer, you'll need to back up your documents somewhere as you will need to completely erase the hard drive in your Mini before installing Mavericks. If you use Time Machine to backup, this could be an option - but I couldn't be certain as I've never downgraded from Yosemite to Mavericks and I know that Time Machine backs up the OS as well, so there could be some issues involved.

Once that's done, you can then bring over your documents and reconfigure your apps (if you have iCloud and sync your Apple app settings, that should be effortless - but I can't confirm as, again, I've never done the downgrade).

If this sounds like a royal pain in the butt … you're right. Yosemite is a pretty big change from OS X, but if you can stand to stick with it that will probably be your best option.