Don't get sad about the past, for it is not sad about you. It has passed - like the cars on the road through a green traffic-light. It has passed like the moment where the check-out scanner goes 'beep' and you walk away with your things.
And if Nietzsche said, that if you gaze into the abyss, the abyss will gaze back into you, just don't dare to look and it will cease to exist. It will know it's place - to swallow, swallow itself - up into space. Just don't look into the past, for it is just a stretched blackness. That is waiting, waiting to take you back, into the life you should have had.