I have the same thing with anything related to art. What happens to me, though, is an idea I love, love, love pops into my head when I am least expecting it and doing something other than art. I suggest you choose a favorite idea and think of a plot with a beginning, middle, and end. If you can't choose, pick some of your favorites and ask a friend to choose one, cause what happens to me is when My friend chooses it, I start preferring another and soon I'll get the right one.