Sad story. As a side note: After years trying to catch my thoughts on a computer, I’ve came to realize that handwriting is much more effective tool for me. My thoughts start flowing once I have a pen in my hand. Now I have a series of diaries that I use with several people, my wife, myself, my late mum.
When you say "with", do you mean you write a letter & other person writes an answer letter in that same diary? E.g. with wife; does that means letters between you both in that diary?
Thinking of it, we had something like that in high school, where (usually girls) would use a single book with her best friend back and forth. We called it "Briefbuch" (letter book) in German.