How do I get over my highway/interstate driving anxiety?

@blindbob Were you in a car accident as a child or teenager. Even a mild one can cause PTSD if you feared for your life or if anyone was hurt. If so, EMDR is a proven effective cure.

Dealing with anxiety and fear requires a specific technique. Rather than tackling it head on, like during a hailstorm, start with much smaller increments. For instance, select a place that looks safe on the map, enter the highway during ideal conditions and then leave it at the next exit. Find a good place to park and just notice the feelings inside you; wait till the adrenaline has subsided, and then return home and give yourself a really enjoyable reward - say a bath, time with a pet or friend. Consider keeping a log where you record how each episode goes. Each day (or once a week), repeat the same exercise until you can do it while remaining calm. The choose other places and repeat, getting on and then off at the next exit. When you can do this at many and unfamiliar places. When you remain calm at all of these, increase the length of your drive and get off at the second exit, still during optimum conditions like a grey or sunny day with no wind or rain.
As you gradually increases your distance, still staying calm, begin to notice how your body feels with the vibration of the vehicle and registering the speed at which the outside appears to approach at pass you. Notice your speed relative to other drivers on the road. Practice remaining one car length behind the care in front for every 10 miles per hour. Pick a marker on the side of the highway and work out how many seconds it takes for you to reach that marker. Make a note of how far you can travel in three seconds. That distance speed is the closest you can get and still have enough time to break if the car or truck in front suddenly slams on their breaks. And everything needs to be 10 mph slower in wet weather;15 if it's raining after a long dry spell because the roads will have collected grease.

Fear is its own reward and reinforcement. Every time we avoid something we fear, not only do we not gain the skills to deal with it, we also cause the fear to grow.

It was unwise for your sister to demand that you drive her during a hailstorm. No kind of high wind or heavy weather is safe. If a hailstone had smashed your windscreen it could easily have caused a fatal accident. In that situation, checking the weather forecast and leaving home hours before the hail could have made it a much easier drive.

If you decide you don't want to be a driver, it might be best to let your licence lapse; then no one can put that demand on you.

@hartfire I never had an accident as a kid, but I was in the car when my mom had an anxiety attack before I learned to drive. I felt trapped and helpless. I used to do the thing where I would get on and get off at the next exit. I lived near an airport so I would do that when I needed to go to the airport. Then I started making trips to the next city over to their airport. I usually did my driving in clear weather, but the bad weather situations were a surprise. My sister didn't force me to drive during the hailstorm, but we were on our way home when it started and I just had to make sure everyone in the car got home safely. I wish I had a place where I could feel comfortable practicing, but now that I live in a large urban city traffic is outrageous, and on top of that we have carjackers and shootings on expressways. I was also in a head-on collision when I was 20 so now I have general anxiety about driving in construction zones, but I still manage on regular roads.

@blindbob Yikes. All of those situations taken together are more than enough to explain your anxiety.
On the plus side, I bet it makes you a very careful and safe driver.

@Vivaci But you have to get over and get back every time someone merges on to the expressway. Changing lanes to let them in always causes me lots of anxiety.