I was abandonened and discarded by my exBPDgf about three months ago with over two months of NC.
We share a similar experience.
These letters are the final thing I need to get rid of ... . But its so hard because they are actual history not only of a deeply intimate relationship but of the person I thought she was and really wanted her to be.
No one can tell you what to do with the letters. I got rid of all of the gifts, packed up all of the photos, family photos for the kids at a later date, birthday / holiday cards, but the last remaining thing were intimate letters. All from the idealization phase about her love.
I packed them up in a box and stored them away. Out of sight, out of mind. Maybe one day I'll go back to them, but I couldn't get rid of it in the emotional state I was in. Maybe I'll revisit them one day.