nogardous: i have a theory about that. i think that ppl that lose weight too fast, say more than 1-2 lbs a month, once the diet/exercise regiment is over, the body freaks out, and gains back all the weight, plus more.
think about how much exercise and calorie restriction it takes to burn enough fat, while building muscle, to lose 1 lb. it's kinda shocking when the numbers are actually crunched. you really can't keep up that kind of exercise and calorie restriction to maintain that sort of weight, especially as you age.
so, modify your diet into something livable. instead of "diet", think about how to eat more healthily. the same with an exercise regiment. you really shouldn't be losing more than 1-2lbs PER MONTH, not per week. that way, the weight will stay off, and you'll not yo-yo.