Ideally it's about balance. You are right that if we spend ALL of our time reliving the past or imagining the future, we are are going to miss things happening right now. But it's okay to sometimes think about the past and future. If you're thinking about a pleasant memory or imagining an exciting future event, you're having a pleasant time in the present moment thinking about those things.

But also, I find that by setting aside some time each day to do some private jourlaing to process my thoughts, I'm more likely to find it easier to be "in the moment" the rest of the day.