I have a question about the eating disorder bulimia?

I really want to stop, but I can't. I mean if I stop purging, I gain so much! Tell me how to have the will power to stop the binge-purge cycle

Seek professional help. Bulimia nervosa is part of the obsessive-compulsive complex of mental disorders. Your problem does not stem from lack of willpower, but rather from an innate need for control over yourself or your environment. A counselor, perhaps in an inpatient setting, can help you explore why you feel the way you do and help you to overcome this obstacle.

It's very good that you have the insight to realize what you're doing is unhealthy, and that you want to help yourself. I wish you many blessings on the long road for recovery.

Have you tried therapy? Eating disorders are extremely hard to get over (think of them as an addiction to drugs) and it's pretty rare for a person to recover by themselves, most of the time they need professional help.

If you don't get help for your disorder, you can have a heart attack, you can ruin your teeth, your espogus will be messed up from stomach acids, you will stop menstuating (if you haven't already), and a whole mess of other problems including death. So it really is important to get help now before these other issues begin to pop up.
First let someone who cares for you know what is going on with you and that you need help. Try eating foods that you know are low calorie and healthy so you will not have the uncontrollable urge to purge. Also eat with your family and have them help you stay occupied for 1 hour after you eat so your body can digest the food. It is a hard habit to break I know I have been there. You do not want to make your self sick trying to stay thin. The women you see on TV and in magazines are not a realistic body image. Talk to your doctor so he can tell you what weight is ideal for you. Then you know what your goal is. If you are heavier cut calories but never starve or purge because your body will think it is starving and actually hang on to all calories or start eating your muscle. If you are less than ideal then but do not want to gain eat 1200 calories to maintain your weight and 1500-1800 to gain depending on your activity level.
Yes, you really need to seek professional help. You can't do this alone and it is not about willpower. You have a mental illness and it needs the correct treatment. Good luck : )