It's okay to look at the past and the future. Just don't stare.
There's absolutely nothing wrong with re-living some fond memories and learning from your past. There's also nothing wrong with planning and visualizing your future. However, don't spend too much time doing either one at the expense of making the most of the present. Focus on today and you'll create many great memories and your future will be bright.
[To him] Jesus said, “No one who sets a hand to the plow and looks to what was left behind is fit for the kingdom of God.”