You need to install a plugin to disable Gutenberg. Either Classic Editor Plugin or Disable Gutenberg is a good choice. There are other methods if you want to research those too. Once you apply one of these you should be returned to what you had before so long as you haven’t modified anything drastically by using Gutenberg.
Bottom line. Gutenberg (new block editor) can be problematic with sites that have been built with older plugins and custom coding. The new block editor also follows a whole new way of working with different workflows that don’t match the way WordPress has mainly worked up until now.
How you deal with this will vary from user to user. Personally all work I do with WordPress will remain using the older editor. I will continue to asses how the new block editor progresses and if it matures into something more usable and less buggy, I will consider using full time on new projects. This could take a year or two, depending on how involved the way you work is tied to the old editor and how easy it is to convert to a new way of working with the new editor.
You probably also need to consider using themes that are compatible with the new editor to avoid any of the quirks that can manifest themselves in a bug like manner.
As someone who is ambivalent myself about the new editor, spending a bit of time, outlining cogently what is amiss regarding it, might drive home where the project is missing some important features.
