Here's another change of perspective I found useful:
I used to place no value on the past. Now, I see that
to have a realistic appreciation of my life, I can't
be so casual about disregarding an ever growing part of it!
Time is past, present, and future. Besides the standard advice of being in the moment, and appreciating the time at hand, people need to learn to value their own past experience, to see the days gone by as little precious items that we peruse and enjoy in and of themselves. (Not that you should 'live in the past' either while you still have plenty of living in front of you.)
Our optimistic natures cause us to always look to the future, to make up for the shortcomings of the past, to have a chance to accomplish the things we've always wanted to do. But our pasts live on, in our selves, and we need to treat ourselves with the respect we deserve.