I'm going to be Catholic...?

So my mom is Jewish and my Dad is Catholic. Since I was born to a Jewish mother, technically, I'm Jewish. The problem is, I just don't want to be Jewish. I never had a bat mitzvah and to be honest, I've always felt Catholic and never Jewish. I believe in some stuff of the Jewish faith, but I think because it's like the base of Catholicism. I believe in almost everything Catholic and I feel like it's just who I am. I'm just not Jewish in my mind. I've been pondering this my whole life and I have decided that I want to be Catholic. But I don't want to have to wait until I'm 18. I'm 14 now and I don't know how to bring it up to my mother. My mother's mother was born Catholic but converted to Judaism to marry my grandfather, and some of her family (mostly her mother) weren't very OK with that. And I don't think my grandfather is going to be OK with me being Catholic. He wasn't thrilled when he found out I was going to a Catholic school...