my heart is true

I've never considered myself to be prone to self-deception. I hope I've been fooling myself:

The psychological evidence indicates that self-deceived individuals are happier than individuals who are not self-deceived (Taylor 1989, Alloy and Abramson 1979, Taylor and Brown 1988). Lack of self-deception, in fact, is a strong sign of depression. (The depressed are typically not self-deceived, except about their likelihood of escaping depression, which they underestimate.) Individuals who feel good about themselves, whether or not the facts merit this feeling, also tend to achieve more. They have more self-confidence, are more willing to take risks, and have an easier time commanding the loyalty of others. Self-deception also may protect against a tendency towards distraction. If individuals are geared towards a few major goals (such as food, status, and sex), self-deception may be an evolved defense mechanism against worries and distractions that might cause a loss of focus (Trivers 2000).

This paper explains a lot, I think.