There are so many diets out there that they will all help you lose weight, but a lot of the time you have a hard time keeping it off. When you eliminate food groups or types of foods you will lose weight, because you are no longer eating them and it limits your options. The thing you need to remember is that you need to find something that you can stick with for the rest of your life. A lot of people can stick with it for the short term but then in the long term it is just to hard, to depriving or very restrictive.
Another reason is that there is typically a reason why you are overweight. If you do not fix the problem nothing will work long term. It can be that you are an emotional eater, binge eater, or just lack of true motivation to continuer with a healthy lifestyle. Until you are ready to fix those issues nothing will last. Anyone can do it, you just need to be ready for it and want to make the changes that need to be worked on.
In my practice I always have everyone make goals that they want to accomplish as far as diet and exercise. With the goals I always make one long term goal with short term goals to help get to the long term. If you just say I want to lose 50 lbs, it can seem impossible to get there. So if you have short term goals that are easier to accomplish, you will be able to meet those goals quicker and feel successful. Examples for short term goals would be eating 5 servings of fruits and vegetables a day or being able to do 5 pushups. Then come up with strategies to get you there. Always make goals quantifiable, otherwise how do you know you have accomplished them.
I always encourage my clients to keep a food journal. It one makes you accountable for what you eat. It also is useful to go back and look at patterns to see where things are going wrong. Are the portions to big? Do I overeat while watching TV? Do I make bad choices when I am stressed out?
The most important is that you have a positive attitude, along with a positive and supportive environment.
Just look at it as a life long growing and learning experience. We all have our ups and downs, it is just learning from them and coming out stronger and better in the end.