I'm glad a thread like this exists on Smogon!
Three years ago (age 17), I was 241 lbs and felt horrible. Even though I'm 6'4" I was still pretty thick, had a bad diet, wasn't great with exercise (though I did get out a decent amount), etc. Then I got mono and lost 10 lbs from not eating since I lost my appetite, but when it came back I gained again and when I hit 241 on our bathroom scale, I had finally had enough after numerous "I've had enough" moments that weren't real.
But this time I wrote down everything I ate in a Wordpad file on my computer. I highlighted unhealthy foods with red, healthy foods or substitutes for unhealthy foods in green, and "neutral" foods (something like peanut butter crackers or chocolate milk) in gray. It wasn't perfect, especially my classifications, but every night I had to be accountable to myself. I also had what I weighed in at each night under it. I dropped the first six pounds quickly--I suspect some of that was water weight or weight that I hadn't entirely "packed on" yet, if I may be unscientific about it--but then stagnated. Even so, I kept with it and steadily had mostly green reports, with the only red thing usually being a sugary frozen snack I let myself have every day or two.
Eventually, I started making progress, as 235 lbs turned to 233 which turned to 229, and once I got into the 220s, the weight started falling off almost effortlessly. I started during the winter (and was still recovering from mono!), so I didn't exercise much, but once I did it only accelerated things. By the NBA playoffs that year (weird I know, but I associate different weights with specific times and I remember weighing 224 lbs when the Mavs-Blazers series was on at a local restaurant, lol), I had made real progress and was feeling great. I was at 217 lbs during the thoroughly depressing Celtics-Heat series and finished off the school year at 212 lbs. By then I had sort of stopped trying--over the summer I exercised a lot and it just kept coming off until around 190 lbs. By then I had long since stopped being so strict with my diet. I could afford it.
Losing a whopping fifty pounds was huge for my self-confidence. Besides looking better, it just felt awesome to finally slay that dragon, as I can remember being self-aware about my weight back in third grade. My target had always been 180 lbs though, which was arbitrary but still a real goal. After a few years of taking it easy and holding steady, I decided to go for it and started running, watching my diet again, and even counting calories. I had a ton of success, becoming a much more proficient runner and trimming down to 176 lbs before getting plantar fasciitis in my right foot, which shut me down not just from running but also from basketball, a major hobby over the summer.
It's still not entirely better, mostly because I've been very lazy rehabbing it. I've taken to using the pseudo-bikes in my school's gym and lifting for my club sport for exercise. Lifting has been interesting--I've always been very very weak, so adding legit strength has been…an experience. Even though I'm still pretty weak, it's really awesome to see incremental progress in much the same way I described above. (Hopefully the details weren't too dull.) Tomorrow night is legs day...can't wait for more lunges...
I would say that living a healthy lifestyle has changed my life in very positive ways. First, the self-confidence from exercising a degree of control over my physical fitness and appearance is empowering. Furthermore, I just feel better, probably because healthy food rarely makes you feel like crap, even though I'm not nearly as strict as I should be anymore. Lastly, a more recent transformation has been that now I feel like I can be a positive influence on my family, especially my parents. I feel especially useful with my father, who is overweight and very much at risk for ailments (or whatever the case may be) that comes with that, whether it be a heart attack, diabetes, etc. Over the past week we have moved all of the cookies and chocolate, unthinkably plentiful around Christmas, out. Baked chips have replaced fattier ones. Soda is being banned. Unfortunately, I'll have to leave soon, but "I don't want to end up like Dad" was a main motivator three years ago and now I hope to be a small positive force for change. I have no idea if I'll be helpful or just annoying, but I hope I be be positive in some way.
Anyway, I hope this wasn't too out of place, but I just wanted to share my experience with the subject matter of this thread. Maybe it will be a nugget of encouragement to anyone who struggles with this stuff--even now, it's still a struggle for me. I suspect it is to some degree for everyone.
(Wow, that was wordy.)