I cannot advise on the letter- but it does suck.
As to anxiety- my first conscious, remembered emotion/ feeling when a toddler was anxiety. For me- a doc for the depression, a psychologist and CBT to work on coping now....and as to the causes? LOTS! I found Adult Children of Alcoholics meetings useful- but not when I feel overly anxious. I have to force myself to be around humans. A good place is at meetings...I have a home group which is small, safe and supportive and has a stable atmosphere.