How do I convince my parents to let me quit dance?

Just tell them that you are not going to do it anymore. Tell them that you had no choice at 5 to start doing all of those things and that if they cared at all about how you feel that they would realize those things are what they want not what you want. If they tell you that you will get fat, tell them that is fine. You can always lose the weight later but you would rather be fat then miserable and depressed for the rest of your life. If that fails, well then the only thing you can do is to move out when you are of age. Check the laws in your state and find out if the age is 16 or 18. Maybe then they will realize you are serious.