Hang in there Diane...
It's not easy dealing with JW family. They can be very frustrating, especially when the subject of the religion is at the forefront.
What helps me is to remember that they are victims. They've been brainwashed by the WTS. Yes, they can be cold and calculating but somewhere deeper inside is the real person.
I've learned also, for the most part, to not have religious discussions, JW or otherwise with them. It only leads to trouble.
I think you feel bad because you lost it, you yelled at someone you've never yelled at before or want to yell at...we've all been there. We all have regrets in how we've handled certain people and situations. While it hurts now, it can be a learning experience. Now that you know how your mom may act/react to this situation, you know now what to look for in the future in order to avoid the argument...even if if means biting your tongue and walking away. That's what I've learned...the hard way. I'm a stubborn SOB and I'll argue for arguing sake, but with JW family, I've realized that sitting in silence is the best thing for everyone. Let them believe what they want, I know what I know. I'll pick my times to drop hints or make simple statemsnt that hopefully spark something.
It still hurts and it probably will for a bit, but it'll get better. Mothers and daughters may fight but there is a bond that is deeper than the one to the WTS in most cases. You can find that common ground and move forward.
Good luck...