microsoft office

I just updated to OS Catalina, but my Microsoft Office suite (Word, Powerpoint, Excel) won't let me access my existing files with full editing access--read-only--unless I enter an annual subscription program. What?

Catalina does not retro-support previous versions of some software, including the Office suite.

So..I wanted to back down my upgrades to Mojave, but first needed to reinstall my original OS X Mountain Lion, which now is giving me problems. I'm stuck at a dead end when it asks me to indicate where Mountain Lion should start up from.

So..I'm operating on Catalina, but this Office suite access thing is a problem....any solutions?