Catalina mail problem: blank emails
After upgrading to Catalina, most of the text in my emails have vanished. Sometimes there's just a blank message field, and sometimes there is a collection of overlapping boxes with question marks. Oddly enough, the images are mostly still there.
This has happened to both sent and received emails, and to messages sent before and after installing Catalina. Occasionally there will be an intact message, but I can't pick out anything to explain why one will be visible and the next one not.
Most of the messages come to a gmail address, and all seem to be intact when I go to gmail on a Chrome browser.
Is there any way to fix this and if not, how can I revert back to Mojave until this is fixed?
MacBook Pro 13", macOS 10.15