Early in the day you can will yourself to do the work that matters, work out, and eat broccoli.
Later on, with willpower reserves depleted, you procrastinate, slob around and stuff crisps into your face.
Help yourself out by creating rules. So when willpower trickles, rules kick in. Like these:
“I don’t eat after 8pm”
“I do 10 minutes yoga before bed”
“I do one hour focussed writing from 9-10pm”
“I only eat crisps on the 29th February, because nothing counts on that day”.
You don’t need as much willpower to follow rules.
(But if today is the 29th Feb, there’s no rules, go nuts.)