I keep coming up with all these great ideas for things to write or draw, and I keep finding out later that my brain stole it from somewhere else. Every time I try to write a story, I go really into detail with the plot and think to myself that 'this is a great idea', but when I sum up the idea, I realize it sounds like the plot from somewhere else, or like the main character might as well be a person I've seen on TV. It seems like every time I think I'm coming up with something new, it's already been used, and I've seen it before. Why does my brain do this? Do I have no imagination? Is there a way to un-train this tendency?