Some of these are bad stuff - times when you were hurt, disappointed, or trod over. It could be times when you did stupid things and had to face the consequences. It's usually easy to let these go because you don't want to remember them. It hurts too much. But at times, it's difficult to let them go as well because you've grown to let pain define you; you've started to think of yourself in terms of what you're NOT. (Or you're just secretly a masochist.)
Then there are the good stuff - the things you've achieved, the accolades you've earned, the great experiences you cling to. You remember these with a little bit of pride and nostalgia and it's hard to let go of, because why should you?
And yet, sometimes it's our own successes that hold us back, because we're so satisfied with what we already have that we stop moving forward. And sometimes, we're so caught up in the memory of what used to be that we can't grasp the new thing that's coming, instead ending up wallowing in the bitterness of "when I was young(er)".
It's not easy, and I still struggle with it.
But at least I am aware of the need to let go.