Hi,
Have you considered PTSD?
I too noticed some time ago, that I had difficulties letting go of family stuff, from my early 20's and what followed... while others let things go, seemingly to me, more easily.
I eventually got on top of it by addressing PTSD ... and since then, you might say, I have overcome this issue. I don't dwell in the past, at the losses and mistreatments any longer, unless there is a profit in it to do so, and as we all know, our histories are important to us, but they definitely do not control us nor define us.
If it's letting go you have an issue with, you might check out PTSD.
If you do not want to start therapy over this, and other issues, like anxiety, panic, what-have-you right away, there are good therapeutic tapes for sale by very qualified PhD's addressing how to positively change our thinking that also helps us shift our mind-set to overcome issues like clinging to the past.
It's always good to know what our past meant to us. Most of us have figured out if any damages were created that need to be addressed, although some people aren't aware for one reason or another, and need to explore their past for more information. .
Understanding our beginnings, one can leave it there if they choose - in the past - because we are going the other way, after all, and are now free to examine our daily responses to situations and people in our present life. That is staying on top of ourselves, which is what most of us prefer to do anyway, right?
Hope you figure out what it is you need to do for yourself to overcome your issue. It's worth working on ourselves, and since we are all individuals, our paths are different also.
Best to you...
Peace and Encouragement,
nightbird