How can I downgrade back to Catalina?

Revert your Mac to a previous macOS version

That or work through the issues in Big Sur >> Etrecheck

Suggest downloading the Application Etrecheck directly from a Trusted Developer and well Respected ASC Contributor. The application is free or paid from added features. Run the application with Full Disc Access ( Security & Privacy - Full Disc Access ). It will take a Snap Shot -  both the hardware and software. The Report will Not Reveal Any Personal Information. Post back the Full Report - copy and paste - using the Additional Text Icon ( 3rd Icon to last )

We can have a look at the report for possible issues and may have possible suggestions to resolve the issues.

Any Third Party Applications that will interfere with the normal operation of the OS, alter, modify, remove or delete or attempt to do so is an invitation for disaster and may require a Reinstallation of the OS.

kanehenry wrote:

Thanks! What if I don't use a Time Machine? Is it impossible?

If you have an alternate backup of your user data, sure no problem

From Recovery you have options you can erase/reformat/initialize as new and reinstall the macOS that shipped with

the mac when new—then you can upgrade from there—

older macOS  installer backups on this server: https://support.apple.com/en-gb/HT211683

verify you are using the Safari browser to download/initiate the macOS links.

How to erase your Intel-based Mac - https://support.apple.com/en-us/HT208496

How to reinstall macOS from macOS Recovery - https://support.apple.com/en-us/HT204904

Revert your Mac to a previous macOS version

https://support.apple.com/en-lamr/guide/mac-help/mh15216/10.15/mac/10.15

3-2-1 Backup Strategy: three copies of your data, two different methods, and one offsite.

Before going thru all the trouble and effort of downgrading the systems you should try to ascertain what is causing the current problems. It'll be a lot easier. As P. Phillips suggested download and run Etrecheck. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support  to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.

IMPORTANT:

Before running Etrecheck assign Full Disk Access to Etrecheck in the Etrecheck's Privacy preference pane so that it can get additional information from the Console and log files for the report:

Also click and read the About info to further permit full disk access.

Copy the report

and use the Additional Text button to include the report in your reply.

Then we can examine the report and see if we can determine what might be causing the problem.

Both the previous persons have provided options to get this computer back and working. As have I in original reply.

It is now up to the User ( you ) to decide what avenue is best to pursue.

If addition questions or concerns about any of the methods arise - please post back and any one of us will be willing to assist.

No not impossible. As previous posting outlined Am away from computer for awhile and will check back later on the progress.