Be very loving and patient with your mom. I had left the religion at fifteen and didn't come back untill I was forty and was in it for nine years and left. Everytime I would visit mom she would beg for me to come back to Jehovah and one day I told her I had to get off the phone or I would say something that I would regret down the road. She asked if I would call her in three weeks and I said yeah what ever. Three weeks went by and I got a phone call saying that she had died. I didn't know she was going in to have her leg amputated and she had taken a bottle of painkillers just before her surgery. While she was in recovery she had a massive heart attack. We found this out from the lady that was sharing a room with her. She said she should have spoken up but mom always talked about not wanting to live anymore. The religion sure can mess with peoples lives.
With what I know now, I would have sat down with her and asked why is she in it and then told her those reasons she picked aren't your own. That your truly searching for answers that your not allowed to question. Reassure her of your love for her and that you won't go do anything stupid or get yourself df'd. Just leave it alone untill she brings it up again. It will become a habit with her to ask but it won't be as shocking as it is when she first finds out that you left. When she starts to hear the happiness in your voice, hopefully that alone will be enough for her to back off and not nag you about it. Also reassure her that you will never lose your love for Jesus or Jehovah. This will tell her you haven't turned your back on them either.
I wish you well and wish I could have my mom back.