'Contentment' is one of the niyamas, the set of five personal observances that Patanjali lists in the second chapter of his Yoga-sutras. It's also a state of being that can be very hard to attain. How do we find real contentment?
One way is to try to understand what the obstacles to contentment are. Two of those obstacles are 1) pining for an irretrievable past and 2) worrying about what the future will bring.