Daddy is not pleased. He does not want Anne to go upstairs and "neck" with Peter.
Anne writes her father a little letter about how her life has not been easy, shut up away as it has been, and she has had to learn to be very independent, without support from her parents. They have not, and do not, help her through her difficulties, so she has had to become older than she really is.
So now that he wants her to be obedient and stop seeing Peter, she feels she should make her own decisions. Either he forbids or he trusts her: there’s no middle ground.