It probably isn't willpower.
I spent ten years assuming I'd just gotten lazy. I hadn't. I'd become insulin resistant, and nobody had ever explained to me what that actually meant.
In college I ran three to five miles a week, played sports, and danced ten or eleven hours a day. At 22 I was 6'1", around 200 pounds, benching 280 and squatting 310. Then I finished school, started teaching, and started working eighteen-hour days. The activity disappeared. The eating didn't change. Nobody warned me what happens next.
Here's the short version of what was going on inside. Every time you eat, your pancreas releases insulin to move sugar out of your blood and into your cells. Do that often enough, with the food most of us live on, and your cells stop responding to the signal. So your pancreas sends more. And more.
High insulin is what makes you hungry an hour after a full meal. It's the three o'clock crash. It's the weight that settles around your middle and won't shift no matter how carefully you eat. Nothing about you is broken. A signal your body used to hear just isn't getting through any more.
Gym memberships. Low carb. Keto. A chiropractor. At one point a pill-based program that damaged my vocal cords. Each one worked for a while, then handed the weight back with interest. What finally changed things wasn't more discipline — it was doing something about the insulin itself.