At OP-
I empathise with you as a agnostic who left Islam at the age of 17 (I'm 28 now). I'm from a Bengali Muslim familiy and while not overly religious, my parents still believe in it all and my mother has become more religious as she's gotten older (as in prays more). I don't speak to my father no more but I'm very close to my mother and I don't really want to hurt her feelings by being so blatent about my beliefs though I wish I could. I'm sure she'd still love me no matter what but I know she'd be quite hurt by it all. She kind of knows (my siblings do) but she's in denial about it all.
Once you become an atheist and see the world from a purely logical and scientific point of view, it's very unlikely you'll go back to any religion. If I were you I'd start planning for your future as an independent person who can look after yourself. Start learning to cook, wash and iron your own clothes, clean up after yourself, save your money, do well in education or your workplace. Then you can move out with the money you've saved and the job you have and tell your parents if you want. If they do disown you at least you have your own place and you know you don't need them to survive.
