Catalina has made my Mac unusable

Please determine if the same problems occur in "Safe Mode": Use safe mode to isolate issues with your Mac.

as a photographer I’m finding it very stressful that all my recent photos hoots could be lost - let alone the pictures of my late mum and dad that are on this Mac.

Are they not stored anywhere else? Have you not considered the implications of a poorly running Mac? Given the symptoms you describe, it's likely its hard disk drive is already operating in a state of failure.

To learn how to use Time Machine please read Back up your files with Time Machine on Mac. If you don't have a backup, you will eventually lose all that Mac's contents regardless of the operating system it's running. Conversely, having more than just one backup virtually guarantees you will never lose anything. Time Machine can back up to as many separate devices as you wish. Storing those backups in geographically separate areas reduces the statistical likelihood of sudden, total and catastrophic loss to zero.

That's good news, and wiping the Mac might fix it, but it shouldn't be necessary.

"Safe Mode" is a troubleshooting aid, so try it first.