Most Helpful Guy
Any particular reason your mom won't support it? That's your decision regardless. She should respect it but if she won't help you then just don't eat it, ie if she's cooked steak, potatoes and corn, eat the potatoes and corn only or make your own meals. Chances are she won't be too offended if you are already skipping meals and she'll know that you are serious about it.
Most Helpful Girl
If you want to become a vegetarian you will no longer have to rely on your mum to cook you meals, and you will have to make them yourself, meal prep would save your time and you can just grab and go. Over time when I was younger my mum started to make things meat free when I was a vegetarian.