Here is a very easy, true fact to acknowledge- bowing to the ego is the opposite of willpower. This doesn't mean this ethereal "EGO" that /x/ is scared of, it is those moments where you subconsciously think "hungry" so you walk to the fridge without consciously, mindfully deciding to. It's when you hit a roadblock in your studies and 'decide' to distract for a bit. It's when you have a conversation with someone but find your eye wandering to a nearby TV left on.
Ego-death (not in the drug sense at all) is willpower. Taking a step back from this base reactions is the ONLY way to gain willpower. There is not a single thing more or less that you can do than by constantly (gently! without ***SHAME***, or else you just fall into a new trap) withholding from whatever "you" want. This "you" that reaches for junk food, turns on the TV, checks your phone, ignores work, puts off chores, etc, is the one that will forever prevent you from growing.
Now that that is clear, the quickest way to establish HUGE progress in willpower? Fasting. Do either day-long dry fasts, or 24-hour wet fasts. Do them often too, at least twice a week. Once you start that, you see how visceral and clear that sudden urge to eat, and your ego-reaction to it, is. As you build this resistance up you will build it up in other arenas as well.
Also, meditate, because if you haven't (and built up a story about how you dislike when people recommend it), you will never get very far.