I forgot my old note password

There is no way to get that back.

If you forgot your notes password

You can unlock a note only when you enter the password that it's locked with. If you forgot your password, Apple can't help you regain access to your locked notes.

You can create a new password, but this won’t give you access to your old notes. But it will let you password protect any notes that you create going forward with the new password. This could lead to you having multiple notes with different passwords.

If you have multiple passwords, you can't tell which password you should use when you look at your notes in the Notes list. When you open a note locked by your old password but enter your current password, you'll see an alert that you entered the wrong password with a hint for your old one. If you then enter the correct old password, you get the option to update that note's password to your current one.