It’s easier to mentally map that the default behaviour undo/redo for Emacs (which is not unreasonable, just complex).

The source for undo-tree contains documentation which very effectively describes the way the library works with examples and comparisons with how Emacs does things by default: https://gitlab.com/tsc25/undo-tree/-/blob/master/undo-tree.e...

I was too at first, until I learned the shortcut keys (to move to the next branch specifically), then everything was fine.

On this point, C-h m yields a complete list of keybindings for all modes active in whatever buffer you do it in.

More generally, for anyone new to Emacs and struggling as I once did, I can't recommend strongly enough that you learn how to use the help system (C-h C-h) and the built-in manual (M-x info). Emacs can teach you a great deal about itself, and these are the ways it does so.

It's easier than it looks! Poke around with the arrow keys a little. You'll get the hang of it pretty quick.