Honestly, I think the letter is a fantastic idea. It should give your doctor some insight into your life that he wouldn't normally be able to get. When I began seeing my previous psychiatrist, I walked in with a detailed history of my mood swings, symptoms, and a brief "personal statement" so he would have an easier time understanding how my bipolar affects me. He thanked me for the information, read through it, and asked if he could add it to my chart for future reference. I imagine your doctor will be happy that you're trying to make his job easier.

I'm sorry about your insomnia and depression. I hope it improves soon.