What are the causes that one couldn't /doesn't want to let go a unwanted memory?

"Memory" is not separate to "self". The concept of persisting self involves its particular experiences which may involves internal restructuring like the water flow restructures the course of a river. The self is the river, not the banks or the water alone.

That's a very nice image and way to explain it- but sometimes I think that some people are more prone to remember the negative while others seem to have an innate ability to focus on the positive and almost instinctively forget the negative. And I don't think it's denial so much as that they are wired toward optimism and positivity and against pessimism and negativity.

And if someone doesn't WANT to let go of a negative or 'unwanted' memory it may be that that memory of that particular circumstance provides that person with a reason or excuse as to why they are the way they are, instead of having to confront the fact that it's entirely possible they could be different if they'd expend their energy on something other than nourishing that negative memory.

I don't mean this to sound judgmental, it's just something I've observed. There are people who have absolutely horrific things happen to them who seem to overcome them and then there are people who have horrific things happen to them who sink beneath the weight of that. I've always been interested in what makes the difference, and I think at least part of it is the way they are wired to or choose to 'remember' their histories.

Gurdjieff gives a picture of "the committee nature" of self. As in Freudian "repression", some facets of "self" can be exiled or banished from the committee room.