Is prolonged sadness considered unlawful (haram), and am I sinful for my past ignorance of this?

Prohibited sadness is the heart's discontent with God's decrees, whether it lasts long or short. However, if sadness is free from discontent, there is no blame for it, because a person has no power to remove it, and there is no difference between prolonged and short sadness. And whoever commits a forbidden act out of ignorance is not held accountable for it, for Allah the Almighty says: "Our Lord, do not take us to task if we forget or make a mistake."