Trying to explain how you feel to your parent about the past is frustrating! Sometimes you can just write the letter and then burn it or push delete. If the family is so messed up (like mine was) they just don't have the capability to understand let alone validate. I think its setting yourself up for disappointment to expect so much out of someone who never had it to begin with.
One thing I've learned in DBT is radical acceptance. Like for my mom, I have to learn to accept that she is never going to change. The only part that can change is how my emotions control me when I am interacting with her or thinking about the past.