The Satcharitra talks about Vidya and Avidya. What's the real difference? I often feel down and think I'm just an unlucky sinner. Is that 'Avidya'?

Yes, you've touched upon a core distinction explained in Chapter 50. The text clarifies that 'Vidya' (true knowledge) is the firm realization of your true nature, the eternal pulse of "I am Brahman," the pure, enlightened, and liberated Soul. Conversely, 'Avidya' (ignorance) is described as the very attitude of thinking, "I am a sinner, unfortunate, luckless." Chapter 50 identifies this negative self-perception as the "mine of 'Avidya'," one of the ancient powers of Maya that leads to a state of bondage, whereas Vidya is the power that leads to liberation.