And there's your problem- a lot of people "diet". Diets don't last forever. Once you come off the diet, you gain the weight back. When they do gain the weight back, they immediately start pushing the new diet, and they try to lose it all too fast, and when that happens, they'll find it just doesn't happen that quickly. You have to think of exercise as a lifelong commitment to a healthy, active way of life. The side effects of weight loss will naturally follow, especially as you change your eating habits and stick with it.
I'm glad to see you want to make those changes, though! It's just how to accomplish it that you need to figure out.
Koffee