I try to eat only fruits, vegetables and peanut butter. I live alone and prepare my own food. I am at the grocery store 5 to 7 days a week. When freshness is essential and overstocking is bad, then I don't know of a better way.
A tip on consuming more vegetables and fruits when you have them: don't also keep other food around that you prefer to veg/fruits. If you prefer x to mangoes and have x and mangoes available to eat, then you'll eat more x than mangoes. Take willpower out of the equation. Force yourself to eat what you "should."
Why did you choose that diet? What advantages have you found from it?
For the past few years my willpower has been steadily increasing. If it lasts and I use it to accomplish something noteworthy, I might write a post about it.
Anyway, what should a rational preference utilitarian with infinite willpower do? Assume that there are no negative effects (unhappiness, stress) with using this willpower, and that they can control their emotions at will.
Clearly they should work a lot more, not spend time on recreation (movies, TV, games), stand instead of sitting, etc..
What else? Should they listen to music? Should they keep their muscles flexed 24/7 ? What should they learn, where would they have the most relative advantage? How much time would be worth spending on social interaction?
I can figure out these things on my own, but those questions are important and good ideas are very valuable.