Sadness - Help

I am in the midst of writing an article on the topic of "Sadness - It's Causes and Remedies". I am already almost 2000 words in BS"D and am discovering more and more that the wisdom of our Sages is ASTOUNDING! If anybody has mekoros that would help me primarily from sifrei chasidus mussar and machshava I would appreciate you sharing them with me. I wont be sad if you don't but will be delighted if you do:-).