Everything worth doing and about half of the things that aren't. If I had to pick one thing, it'd be my weight. I've lost 150lbs and have been keeping it off for years. People who've never lost a significant amount of weight (north of 50lbs) probably can't understand what it is like to have to fight your body every day of your life. I have to avoid temptation as much as possible, which means keeping only enough food…
https://www.ncbi.nlm.nih.gov/m/pubmed/22327054/
Chances are you are underestimating your intake, or overestimating how much you burn through exercise. I don't mean that as a personal critique: most people are terrible at that kind of estimation.