The excerpt in question appears in chapter 8 of The Vanishing Half. In the e-book version, it's on page 846; in the PDF version, it's page 162.
When Stella started passing for white, she couldn't believe how easy it was. In fact, it seemed so easy that she couldn't understand why she'd never done it before. Back then, she deeply resented her parents for not raising her as white. Had they done so, then Stella firmly believes that she would've been spared from white people dragging her father from the porch and the unpleasant sight of baskets of washing cluttering up the living room.
Stella was further convinced that, if she'd been raised white, then she could've finished school and graduated at the top of her class. Maybe she would've gone on to attend a really good school like Yale. And maybe she could've been the kind of girl that Blake's mother would want him to marry.
But that was then, and this is now. Back then, Stella was too young to realize that life was never as simple as she thought it was. It was only later on in life that she came to realize that it takes a long time to "become somebody else" and "how lonely it can be living in a world not meant for you."