Sorry to hear that you're going through such a time...I went throught exactly what you did years ago.
Honestly, I would sit your mum down and just tell her outright what you said here. Tell her that you love her and your family and that you will respect their beliefs but that you have decided not to adopt them yourself. Assure her that you're not going to bust out on some major criminal, immoral spree, but that you just want to choose a different path. It's going to be difficult, especially with all of the people around you, and there may be emotion. Try to stay calm and stay true to yourself. I did...that was 25 years ago, and I don't regret it at all. Think about it and decide for yourself. You're very young, but very well spoken and you strike me as very intelligent.
You can email me anytime if you would like to chat to someone.
Stay strong....BIG HUGS.
Diana