An inability to log in due to an allegedly incorrect password has been associated with startup disk corruption. That means the Mac won't let you in even though you are 100% certain the password is correct.
Passwords are case-sensitive (obviously). The Caps Lock LED as well as the ⇧ symbol in the password field have also been known to indicate an incorrect state upon startup. Press the Caps Lock button (once) to toggle its state and try again, ignoring their status.
Despite the implications of its title follow the steps described in If you forgot your Mac login password - Apple Support. If you get to the end proceed with If you can't reset your Mac login password - Apple Support.
Follow each of those steps all the way through to and including their conclusion.
Let us know what happens when you arrive at that point.