Letters can be very touchy. They can be cathartic for the sender but traumatic for the recipient; cathartic for the sender but meaningless for the recipient; meaningless for the sender....... and on and on.
It is a big risk that a letter like this will burn all remaining bridges. It sounds like this was a risk Bragi was willing to take - the exercise was healing enough to risk the sending, and it doesn't sound like there were undue expectations of a specific outcome attached to it (i.e. "if I send this, she'll feel THIS") That never works out quite right.....
Wascally, I sure know how you feel about your mom :ghug