Occurs to me if it's true, as Jiddhu Krishnamurti says, that all fear is time-projected, that is we fear something that happened in the past will happen to us again, or we're afraid of something that might happen in the future, then this ties in with the presence of dhukkha. A low-level of anxiety is always in the mind, even when we think we're at peace, and as long as anxiety is present we can never be truly in the present. Dukkha itself prevents us from achieving this; those niggling thoughts arise with dukkha as the object.
So pursuing this train of thought, what if one were to use the arising thought as the object of meditation? Would you gain insight into the nature of the cause of that particular moment of anxiety? Confront Mara face-to-face.
Author of Redneck Buddhism: or Will You Reincarnate as Your Own Cousin?