Make it a rule of life never to regret and never to look back. Regret is an appalling waste of energy; you can't build on it; it's only for wallowing in.

When one door closes another door opens; but we so often look so long and so regretfully upon the closed door, that we do not see the ones which open for us.

Fear not for the future, weep not for the past.

Regret for the things we did can be tempered by time; it is regret for the things we did not do that is inconsolable.