We can't let go of the past... because we have a natural tendency and a cultural bias towards the past and the future (read regret and worry).
Individually and collectively we fret and worry about the future be it later today, next week or next year.
Individually and collectively we also re-run the past in our minds constantly. Rehashing old arguments, conversations, actions of ourselves or others and wishing it could have been different. Getting angry or hurt all over again.
The antidote? Acceptance of 'what is' (easy to say more difficult to do) and making conscious attempts to focus on right now. It can be what is wrong with right now or what is right with right now or just looking at right now for 'what it is' and making conscious decisions about how to live with that.
This is a deep question and a short answer but held within it are, I believe, the seeds and keys to an answer. If you disagree, that's OK. Have a really wonderful day - you might as well. :-)