I believe a relatively low carb paleo diet helps, but what we've got to remember is that a lot of us have good reason for what we feel. We may have the scope to change our diets, but how much scope do we have elsewhere? The never ending hymns to 'be positive' and the medicalization of feeling down in a broken society- this is laughable.
I noticed when I started eating liver I felt a pleasant sense of not caring particularly much about what people thought about me.
I also realized the external metric was wrong, for in no small part, that metric is part of an attempt to keep the whole scam that brought us stuff like the SAD (and our little cubicle jobs) in the first place. It is quite possible one's feelings are entirely appropriate to one's situation.