You live longer once you realize that any time spent being unhappy is wasted. - Ruth E. Renkl
Wasted time is just terrible. One of my favorite lines from The Phantom Tollbooth is "It's bad enough wasting time without killing it!" I think there are some unhappy moments that just simply cannot be avoided. Our unhappiness represents a sense of loss, and it's okay to sit with that for a while and allow ourselves to adjust to our new life. After a while, though, it's imperative that we move along, rather than risk wasting whole chunks of our lives. There are wonderful things out in the world. It'd be a shame to miss them.