Well, if you're a pessimist, between a pessimist and an optimist, what could make you sure the pessimist is right? It's because that's how you feel. But what makes you think your feelings are right? Feelings come from thoughts, and are you sure your thoughts are right?
Feelings and thoughts create dynamos. From thoughts, feelings, and from feelings, thoughts. You cannot intellectually change your feelings directly, but thoughts you can interrupt and challenge, and this creates a new feeling.