I do not actually have the book in front of me, but I believe the quote is a variation of the sentiment that one never fully appreciates something until one is about to lose it. The speaker is evidently about to leave home for someplace new, and is looking at her longtime abode with regret that she must leave it behind. In this moment of leaving, she is appreciating her old home, noticing things about it that she loves and will miss, things which she never realized the value of until now, when it is too late to enjoy them. The speaker is wondering if it is common to feel this way.
The feeling that is expressed might be compared, for example, to a student going away to college for the first time. She is happy to be leaving and is looking forward to being out on her own; in her impatience to be starting her own life, perhaps lately home has seemed to be a place of frustration and restrictions, a place from which she can hardly wait to get away. At the moment of her leaving, however, she sees home in a different light; while it has seemed to be a place she has outgrown, at the same time it was a place of security and happiness. The student realizes only then how much she loves her home; in the anticipation of moving on, she had forgotten. Now that she is leaving, it is too late to appreciate these things she should have recognized, and so in a way, her parting is bittersweet.