Just don't carpool.
Say "I'm sorry but this isn't working for me" - you don't need to explain yourself. You don't owe her anything.
I had the same thing a few years ago. One person that I pooled with... he is a nice enough person, but OMG, sorry! - he gets on my LAST FREAK'N NERVE! - I just couldn't be in a car with him 1 hour a day 5x week ... so I opted out of the carpool. It costs me more money in the long run, but my sanity is worth more.
So yeah, just don't carpool.