Personally, I am an introvert and can drift towards melancholy if I'm not careful. Growing up in a positive family, I have learned to reply with 'fantastic' when asked how I'm feeling--even when I feel the opposite. The result is that I genuinely feel that I am 'fantastic' or, at least, positive most of the time.
Now that I am in the world of authorship, I am facing the reality of selling into a marketplace of one million new books per year. I am confident that I have something to add to the 'great conversation' that is humanity but the slow sales can be demoralising.
Then I look outside and the sun is shining, the rabbits are nibbling on the grass, and I realise that my book sales are irrelevant to the world and my happiness. That, however, is a cerebral realisation. My heart still yearns for more sales!