Future you is a stranger, and we seldom empathize with strangers distant from us.
To improve your future life (by having discipline now), you must empathize with your future self enough to make them real: make them a vivid companion, full of your hopes and dreams, a friend that you talk to daily, writing letters into the future, and receiving them back.
Your imaginary friend who makes you better should be your future self, not your past self or your present devil.