downgrade from Catalina to Mojave
how do you downgrade your software from Catalina back to Mojave?
Check it to be sure: boot macOS Recovery, choose "Restore from a Time Machine Backup" then choose the Time Machine disk. A list of available backups will appear.
Nothing will happen to your existing system unless you proceed from there, so you'll have an opportunity to exit if you really don't have a Mojave backup.
Mojave will appear as version 10.14.x, ideally 10.14.6 which was its latest release.
Refer to Recover all your files on Mac for instructions.