Mail suddenly is not working, is Catalina the problem?

I've never had problems like these with Mail.

When I click on a message to read it, the unread status doesn't change, even after I move to another email - which also stays as unread.

I can't delete some messages or they reappear in the same mailbox as unread. This applies to my iMac also, same problems.

When I'm trying to deleting ONE of these reappearing messages, I'm asked if I want to delete SEVERAL THOUSAND - which it doesn't do anyway.

The counts on unread mail are sometimes huge, way more than I have.

I've tried restarting computer and Mail app, rebuilding mailboxes (how do you know if it's being re-built?). Please help! I've never had as many problems with any app; I think Mail might be consuming a huge amount of disk space because I'm always being warned that I'm running out of open space. Thanks!