The past cannot be changed (we'll leave it at that) rather your not acceptance but.. realization of it and an imagined future that is not already incredibly likely to occur will not happen if you do not work toward it now.

Basically. Do you know of someone else who rested on their laurels their whole life? Precisely.

Mindfulness in the sense it’s currently used has never been associated with philosophy as such. It originates with popularized Buddhist meditation practices. Probably the book which kick-started it was Mindfulness in Plain English by Ven. Gunaratana who teaches in the US.

The idea of mindfulness is to be open to understand things as they are. Mindfulness is being in a stable state where you think with your whole mind, to be ready to see and observe, to be ready for thinking.

I think the idea of always living in the present is a bit misleading. To me it means clarity of thought at any time. The nostalgia and pain that comes to you from thinking about the past is real. Mindfulness is understanding your experience of that. Those feelings are happening in the present so they are real, they’re not the memories themselves. It’s not that you’re living in the past, it’s that you’re experiencing nostalgia, heartache and pleasure as a result of thinking. Mindfulness is understanding what’s happening to you.

I'll just point out that to the extent that it is a practice or a habit, it is not present but a repetition of the past.

Having said that, I think it is intended to encourage folks to live in the world rather than in ideas, thoughts memories and imagination. Not that one should not remember or learn, - even philosophers and gurus are not that daft - but these things are provided in order to enrich our immediate experience, not to substitute for it as rather too often happens.

I don't want to forget about these treasured memories. It makes me feel more deeply about life. — Ross Campbell

Live with memories, but not in them. Don't forget the dead, but live with the living.

I'm in a state of utter befuddlement for the following reasons:
1. Don't cry over spilt milk (ignore the past)
2. Don't count your chickens before they hatch (ignore the future)
3. Those who forget history are bound to repeat it (don't ignore the past)
4. The man who fails to plan, plans to fail (don't ignore the future)

Behavior is best when accounting for the past and the future. Learn from the past and plan for the future.

Experience is best when in the present. Don't dwell in regrets or fears.