Juliet apologizes to Mark for how their phone conversation unfolded last night. She says that it's true that she doesn't want him to visit, but it has to do with Elizabeth's death, not him. Juliet writes that she knows he's probably wondering why Elizabeth's death has anything to do with him, but she says that she's mourning.
By standing up to Mark and forbidding him from coming, Juliet shows that she's now more loyal to her friends on Guernsey than she is to her romantic partner. This is one way in which Juliet begins to assert her independence more substantially going forward.