Yesterday is gone, and tomorrow is not promised. All we have is the present, so treat it as the present it is. Seize this very moment and be mindful of being here NOW. It might not be a pleasant moment, or it is one of the most beautiful seconds of your life. Regardless, it is all that you have. Make the most of it.
That does not mean that you need to DO anything, it can simply mean to just BE. To feel what you are feeling, and not escape into yesterday's regrets or tomorrow's worries. Be in the NOW right NOW. This is all that we have ... The time is NOW.