Why did Anne write a letter that upset her father immensly? What did Anne learn from this?

Anne wrote a letter to her father in order to explain her relationship with Peter and to assert her independence, as well as her disappointment that he was upset with her. From this letter, in addition to her father's reaction and their subsequent talk..... Anne learned that words hurt, even word are that written but unspoken.

I should be deeply ashamed of myself, and I am. What's done can't be undone, but at least you can keep it from happening again.