Sometimes it can be difficult to let go of hurts from the past making it hard to move forward. There's also those who refuse to grow up and want to re-live their past (usually through their kids ... just think of TV reality shows like 'Toddlers & Tiaras' or 'Dance Moms' ... enough said). How we remember the past can affect what we choose to hold on to and what we choose to let go of.
There's the present; living each day for the unique beauty that it presents. That could be a baby's laugh, a double rainbow, or getting on the scale and realizing that you finally lost 5 pounds. Enjoy all the little things that are uniquely 'today'.
Then there's the future. The future is the ultimate unknown. It holds uncertainties, lots and lots of questions, hope, and often stress. The point is you can't control the future ... only your present.
Try to live today as if it were your last. There's an old saying 'Don't put off til tomorrow what you can take care of today'. Could that be any more accurate? I kind of think it says it all. Live today with all the hope, strength, ambition, and focus you can muster and it'll get you where you need to be tomorrow.