Actually, she covers the "remember my last" part in Book 7. Remember that Petunia had actually written to Dumbledore as a child, begging to be let into Hogwarts. They had corresponded before. Dumbledore's refusal -- no doubt gentle, but final -- was what set her down the path to a jealous rejection of all that the wizarding world represented.
Petunia would be the poster child for how envy can consume you to the point where you're so busy hating somebody else for their life, that you forget to live your own.
She's kind of like a Muggle version of Aberforth, I suppose -- both she and he were eclipsed in all respects by their stellar siblings. But whereas Dumbledore acknowledges his brother as the better man, nobody could put Petunia in that position.
As always in the HP world ... it's our choices that define us.
Except for the fact that a letter came with Harry when he was dropped at the Dursleys. Right?
Or am I misremembering?