Losing 10 pounds by Sept 23 is not a reasonable goal, as that's too much weight to lose in such a short time.
Most likely the reason you can't stick with a "diet" is because you're cutting your calorie intake too severely. Nobody can stick to a diet in which they don't provide enough food. If you provide adequate calories of healthy foods, chances are you won't have any trouble losing the weight and keeping it off.
Talk to your parents and enlist their help, and possibly that of your doctor in order to make sure you are losing weight in a safe manner. Your body isn't developed fully yet and losing weight in an unhealthy manner now, can cause health issues as you get older.
Exercise is also important when trying to lose weight.