My brother used to have severe panic attacks. He once devised an ingenious way to stop it.
All you can have to do is clear your thoughts. Just blink your eyes and clear whatever thoughts you have in your mind (leaving you a mindless zombie state). If your panic thoughts start rushing in again, blink your eyes again to clear all thoughts.
And as you do it, relax every single muscle in your body - particularly the abdomen and shoulder areas. As you relax, keep blinking your eyes to clear your thoughts. Make sure there's not a single thought or voice in your head.
Of course, your mind will start bombarding thoughts like, "NO WAIT, THIS ISN'T WORKING AT ALL!!!" and such... blink your eyes again. The reason why you're still panicking when you're trying to relax is because of all the adrenaline hormone being released by your adrenal glands and it is circulating around your body - it takes some time to let it die off. So when you stop all your negative thoughts and relax your muscles, you've successfully stopped the adrenal glands from releasing any more adrenaline... it is the previous influx release that's still circulating that's causing you to panic. So blink your eyes and clear your thoughts, relax your muscles, and wait for the adrenaline that's circulating in your body to die off.
Also, try to use CBT (Cognitive Behavior Therapy). I'm sure after your panic attack incident, you're afraid to get back to driving. Now just picture yourself leaving your home and getting back to your car. As you visualise yourself driving back to the highway, think of relaxing and positive thoughts... like the beautiful weather, your favourite song playing on the radio, the tranquility and peacefulness as you drive, .etc. Let yourself know that there's nothing to worry about... you're in control and you're the driver, there's nothing wrong will happen. Then slowly expose yourself to driving back again but take it slowly - like taking the slow lane and cruise slowly, .etc.
Good luck!