It’s what Zeff said, “Biting his own spear of grit for no good reason” destroying the raid suit was a good start because it was one of the most recent times when Sanji chose himself over others. At the time he thought he’d be stronger with the suit than without so when he destroyed it, it was purely for him.
We will need a better explanation for Gabans words, but I think Sanji will need to realize that him sacrificing himself may save the others, but that’s not helpful to him or Luffy. So he’ll need to figure out a way to save the kids, help the giants, and defeat Killingham, all without sacrificing his ideals and dream which will probably be the moment he awajens