Samantha,
One of the biggest reasons for failure to lose weight and maintain a healthy weight, I think, is that most folks try to diet but change nothing else in their life. Exercise is essential, but it's just as important to live a different life. I'm lucky to be a recovering alcoholic, so after my last drink and my last cigarette, I moved onto losing weight. I drank alcohol to change the way I feel. I ate bad stuff for the same reason. I couldn't stay sober and I couldn't maintain my new weight (70 pounds lighter) unless I figured out how to change my feelings in other ways. When I'm anxious. I have tools to use instead of eating ice cream. When I'm depressed, I can do other things rather than binging on Snickers. You lose weight, I think, by taking in and burning the amount of calories to maintain your target weight. You do that and maintain it by learning to react to life differently.
Good luck!
- Ed