What happens? First, your body will think it is being starved. This means it will stop burning calories - everything you put in your mouth will be converted straight to fat cells. Your body does this to protect itself.
Eating disorders WILL make you sick and CAN lead to death, whether you want to hear about it or not. If you develop anorexia, your life will be dominated by calorie counting, lying to friends and family, and developing serious health problems.
If you are genuinely overweight or want to feel better about yourself, there are better longterm ways of doing so. One is a healthy level of exercise. This will tone your stomach or other areas, increase your general fitness, and get you in a good lifelong habit.
Eating disorders happen when you think you are controlling the food, but one day you wake up and realize that the food is controlling you.
Will your boyfriend find you attractive when you become emaciated, lose your hair, suffer constant dizziness, have irregular heartbeats, grow a fine layer of downey hair all over your body to keep yourself warm, and love your disorder more than him?
I'm sorry to say, but unless you have a positive relationship with your body/self, you're not going to be able to have a healthy one with anyone else.