What should I do if I forgot my Notes password?

That's most unfortunate and not something anyone, including Apple can help you with. At the time you created the locked notes password, you were offered the opportunity to create a locked note hint. Did you? The reality is the password you created is fully encrypted on your phone and your phone only. You can guess as many times as you want, but until you remember or enter the correct password, the notes you locked will remain locked.

Passwords which protect important things should be written down and kept in a safe place for future reference.

You're welcome. This is hopefully a lesson, which won't be too painful and you going forward write your passwords down somewhere safe.