We may live in the present, but our mind does not necessarily think this way.
Up to 50% of our time thinking can relate to our past, or imagining the future. Our minds do not live in the present moment.
In fact, our mind is wired to remember better negative experiences than positive situations (1).
It is not a surprise that we may feel unhappy or experience regrets by thinking about the past too often.
But why do we have our minds wired this way?
(1) https://www.webmd.com/brain/news/20070829/bad-memories-easier-to-remember