Yesterday my mom noticed (at a funeral) that I wasn't praying or participating in the mass. She confronted me about it, and I told her that no, I am not Catholic. Apparently it's sinking in and she's a bit hysterical... crying and screaming that she doesn't know me anymore.
What do I do? I don't know how to react/behave when she's doing this. It's like she wants me to feel like I'm doing something wrong, but it isn't working, so she's getting hysterical.
*edit*
I gave her a hug when she calmed down and told her I love her. That seemed to help, a little. Based on her previous behavior in situations where I've done something "wrong," she will (in the future) make barbs and slight passes at my beliefs. (Already she made one: insisting my love of science is causing my social anxiety disorder.) The advice given in the comments is really helpful. I plan on making the most of it.
You know, this is one of those cases (coming out as GLBT would be another one) where we sometimes have to, in essence, parent our parents. Be the patient grownup while they have their temper tantrum, and after they calm down be willing to forgive the hurtful, ridiculous things they said. I think it's more than reasonable to say you'll only talk to her about this when she can be at least calm about it. Encourage her to ask you questions and answer them honestly. Reassure her that nothing about your relationship with her has changed -- she has no need to feel that she doesn't know you.
If this is really a shock to her, it might be a while before she can get used to it, and again, you have to be the patient grownup during that time. But she will probably get used to it eventually. And if after a reasonable length of time she is still giving you grief about it and making it clear that she doesn't accept you, you can let her know that she needs to hurry up and get over it or else she will not see you as often. (All this is entirely parrotting Dan Savage's advice to people whose parents don't accept their sexual orientation: as he says, the only leverage you ultimately have over your parents is your presence in their lives.)
I'll add this: in your conversations with your mother, this is not the right time to argue for the factual correctness of atheism. Even if you don't really believe this, I would emphasize that religion is a very personal matter and that you are just the kind of person to whom religion doesn't seem right. That way you're making it about you, not attacking the foundations of her own beliefs. (Furthermore, this can help reassure her that she didn't fail as a parent -- you were just not the kind of person who could have been given a Catholic education that would really stick.) Ultimately, having close personal relationships with people who you really disagree with about religion has to involve agreeing to disagree and to compartmentalize some things, and also at times to leave some topics off the table for discussion. Obviously, this isn't the way we'd behave in a society of pure rationalists, but the fact is that we do often want to have those relationships and so allowances must be made.
Lastly: you've done the right thing by -- and sorry to keep using this metaphor -- coming out of the closet. Society as a whole is bettered when religious people think of atheists not as a faceless, scary group but as a group of normal people including their own friends and/or children.
Probably not the best time for factual correctness, true. I'll try what you suggested.