I think Woolf means the real emotions you’ll have about the lived moment present, will only appear when this moment becomes past. I think she chose to begin her memoir this way to show us that she didn’t understand what she should’ve truly felt in those moments until she looked back at her past. I totally agree with the quote and in fact, I believe that’s why most of us live in the past, because it brings us a true emotion, like sadness or joy, that we can’t feel in the present.