Sorry, but I'm not sure what else you can do. Either you're misremembering your password, typing it incorrectly (make sure CAPS LOCK is off), or there was some kind of major glitch with the Private Mode functionality. As far as I know, there isn't a way to recover your password (since that would make it a poor security feature). You'd have to factory reset the phone in order to set a new Private Mode password (which unfortunately means you'll lose that data).