What should a christian do if they are not to dwell on their past or worry about their future?

Looking back is interpreted as a longing to go back to the misdeeds of the past and we should not do that. Learn, and do not forget, but dwell not in the past, it is doom. Worry not about the future, for tomorrow is promised to no one. We know not if tomorrow will even come. Worry will not add one second more, but may in fact subtract from ones tomorrows.