The past is done and dusted: if I switch off past thoughts, regrets, resentments, the past can no longer harm me.
The future is out of my hands: if I switch off fearful and anxious thoughts of what might be, the future cannot harm me in the present, now.
The present is mine: to do as I see fit and if I handle the present properly, I'll mould a better future, plus leave behind a better past, that I don't mind remembering.