Kristen Stewart is reportedly planning to write an apology letter to Rupert Sanders' wife.

The 22-year-old actress—who made a public apology to her boyfriend Robert Pattinson yesterday (July 25) for cheating on him with the director—is seeking forgiveness from British model Liberty Ross in private, as she "didn't feel it was appropriate" to include Rupert's wife in her statement.

A source said: "Kristen didn't feel it was appropriate to issue a public apology to Liberty once the story of her affair with Rupert broke.

"Instead, she's going to do the decent thing and write Liberty a private letter expressing her deepest regret for her actions.

"She was thinking of including Liberty, Rupert and the kids in her public apology—but Kristen was so desperate to save her relationship with Rob she decided to concentrate on that first."

Kristen feels "awful" about her illicit affair, particularly because Rupert and Liberty have two young children, Skyla and Tennyson.

The source added to RadarOnline.com: "Kristen feels she made a terribly naive mistake and will do anything to make amends. The fact that Liberty and Rupert have two kids has now dawned on her and she feels awful for what she did."

Kristen described the relationship as a "momentary indiscretion" in her apology yesterday.