Nostalgia has its place but sentimentality must never be trusted. Looking back and feeling sorry that things aren't how they used to be just gets you stuck. The past can only be lived but once. Once it is gone, the only thing to do is move on and experience the now. Sentiment is what should be striven for. Present feelings, not an indulgence of memories of emotions.
It's like that song, "I loved you yesterday and yesterday's gone."