The Alter of Naught is where the giant door is at the Final Battle
You know where in certain areas, Donald and Goofy leave your party, and they're just walking around and you can talk to them?
This is called a 'conversation pit', and the neat thing about these things, is that if you enter one while in a drive form, it forces you out of the Drive Form, and auto-matically re-fills your Drive Gauge
So go to the Alter of Naught, leave down the stairs, go into a Drive Form and if you didn't go into Final Form, just go back and you will be out of your Drive Form and your Drive Gauge will be re-filled
So leave again, go into a Drive Form, and if you didn't get it, go bacl. Keep doing this until you get it