The current Atlantic magazine has quite an interesting little essay arguing that Lincoln's depressions made him a better president; that in struggling with his condition he reached a point of self-understanding and self-mastery that enabled him to well cope with the pressures of office during Civil War.
The article is too short for what it tries to do but points towards some interesting territory. It is also very attuned to Lincoln revisionism and hostile towards those scholars Gore Vidal calls members of the Lincoln priesthood.
The Atlantic's website can only be browsed by subscriber - you'll need hardcopy to study the author's points.