I'm sorry you're experiencing this. It isn't fun.

I don't know if you do relaxation regularly, but I have found that doing relaxation CD's regularly is really helpful in keeping anxiety low. That also helps when something causes anxiety to increase suddenly like when driving because the anxiety level was low before the increase so it doesn't shoot up to unmanagable levels.

A simple exercise of breathing in to the count of 6, holding that breath a few seconds, then breathing out to a count of 8 may help in the moment as well.

If you can't do that, I wonder if grounding techniques could help (there are some posted on the Psychotherapy forrum). Pulling over, looking out the window and focusing on a tree and telling yourself where you are, what you are doing, where you are going.. might help settle and refocus your mind.