How to tell my pressuring mom that I'm agnostic/atheist?

Well I'm a 13 year old guy who has been raised in a Christian family. I used to consider my self a saved Christian along with my mother and my little brother. My mother can sometimes be narrow minded about many issues, though she is a smart women. She prays everyday, reads her bible, sometimes makes unnecessary judgements, etc. The bottom line is that she is a devout Christian (southern Baptist). I am considered to be a good, respectful, intelligent, creative, fun, funny person. I thought that I was a saved Christian, but I began to question Christianity for several monthes and have came to the result of agnosticism/atheism (for a VARIETY of reasons). And no, this is not out of rebellion or for any shock value, I still love and respect my family and friends with all of my heart. One of the challenges of this question is the fact that my mother has a very negative view of atheism, considering that she has had terrible experiences with people who were atheists. The second problem is that lately, she hasn't considered my "saving" to be legitimate and she has been indirectly pressuring me to be "born-again" via pressuring invitational periods in church sermons. But lately I have been silently backing off of these sort of things. And the fact that I haven't "validate" my "saving" has been emotionally wearing her down. This in turn has been hurting me emotionally to see her in pain like this. So it's getting to a breaking point, each church service is increasing this painful pressure, I need to do something NOW! The problem is that I'm scared of how she'll react if I confess my non-belief, but if I don't confess, then this pressure will increase and worsen. Please help me, if I tell her, then she will probably arrange for the preacher the confront me in an intimidating manner (which wouldn't turn out too well). So if I do need to tell her, then HOW do I need to approach and tell her my non-belief? If I shouldn't tell her, then WHAT should I DO?