How do I learn to ignore this girl?

She is ANNOYING, and she hates me because I'm vegan. Whenever she hears me say I'm a vegan, she says "you're selfish because God put animals for us to eat" I always say that's bull crap, and at least I'm not ingesting hormone injected dead animals We were auditioning for roles in The Nutcracker at our dance studio, and I am underweight but NOT anorexic. She's average weight. She looks at my application and goes "OMG, they don't want you, you look like you're about to die. You should weigh more then 100 lbs. at 5'8 sicko" And I told her to shut up. I feel immature, and later apologized and said it was unnecessary on my part to tell her that. But how do I control this in the future so I don't act immature like that?