It seems we have many different ways of experiencing our lives.

I understood Ms Cordova's thought experiment to be (in short) "If you're unhappy today, imagine you're dying and you have one chance to come back and re-experience today."
(Today is the one day we can revisit before it's too late!)

When I'm gloomy, it almost always because I'm in a judging mood, judging myself, or my neighbor, or the world...and mostly myself.
Thinking of my past, even happy moments, doesn't make me happy. For me, memories are not here, they are away, vaguely objective, not real. I can't have them back.

If I re-start my day, it's usually by relaxing and enjoying the odor of lavender, watching my wife groom her dog, or thinking of a piece of music I love, and such. Nothing's changed and everything is changed. I've gotten out of myself.
I've not always been this way, but every day is a chance to get better at forgiving.
I'll try today.