I want to lose 15 pounds in 3 months, how can i stick to my diet?

Only you: Only you can do it since no one else can do it for you. Dieting alone will not be effective in the long run, you need to pursue a regular exercise program including both aerobic and weight-training in moderate degree. Allow yourself one day a week to indulge on the diet. A vegan diet or high protein will likely work better if you can handle it. Carb will be your worse enemy.