Diane does not have psychological disorder, she is suffering from depression and anxiety.
Step-by-step explanation:
Diane being an only child she couldn’t share her feelings with others. Diane seems to be very sensitive. So after getting divorce from Don she was undergoing great depression. She had to shift to smaller apartment and get into job which she doesn’t like.
She could not tolerate the sudden changes that happened in her life. This led her to anxiety. She was continuously thinking about her past and cries for that. Since she was always thinking about the difficulties in her life, she couldn’t sleep peacefully at night.