On earning gotten from forbidden knowledge

From my student days, I have been repeatedly copying materials to learn about some subjects. Since I discovered that this is haram, and I am really afraid of the question, "Where did your wealth came from?", this makes me worried. Because the knowledge is in my head and there is no way to remove it, I am afraid of getting earning from this. Not to mention, there is an obligation to apply the knowledge. What shall I do about it? I can't just seek forgiveness and pay back to every author. I don't even know them all, if I did, I wouldn't remember them all.