How to convince my parents to let me be vegetarian?

I want to become vegetarian for a lot of reasons which I won't bore you with here. My mum doesn't want me to be vegetarian, though. She's a vegetarian herself, but she doesn't want me to be one yet. This is mostly for 2 reasons - I am recovering from an eating disorder and she doesn't want me to lose weight, and she's worried that I won't get enough iron and protein. What do I do?