Downgrade my Mac

Well that was a very bad oversight. You have a long road in front of you.

Create a back up now, on an external disk.

Did you ever make a bootable USB installer with the Install macOS Mojave.app

Download macOS Mojave, How to upgrade to macOS Mojave - Apple Support

Go to section 4 and click on Get macOS Mojave.

Once the download has completed you will find Install macOS Mojave.app

in your Applications folder. With this you can create a bootable USB installer,

using the createinstallmedia command, How to create a bootable installer for macOS – Apple Support.

Having created the bootable USB installer, now Restart your Mac, whilst

pressing and holding down the option/ alt key. When the Startup Manager appears

select the USB from the display of bootable disks, press Return.

You are now going to erase your drive so you had better have made a back up.

Open Disk Utility, click View in the menubar and select Show All Devices,

now click on the Disk, not either of the two indented Volumes

click Erase, give the Disk a name, Format should be APFS and Scheme

should be GUID Partition Map, click on Erase.

When that is done you can start the process of installing Mojave.

After the install is complete you will go through a few set up screens, at one point you will be asked

if you want to Migrate data over, you want to migrate your own data and apps, do not migrate any

system files or settings as these will put Catalina back on your Mac. Read everything twice and then

again until you know exactly what you are doing.

Ok lets hope you made a back up before you upgraded to Catalina.

Revert your Mac to a previous macOS version - Apple Support