So instead of a windscreen, let's work on you. It sounds like what you need more than anything, and that's cool. Based off of what Lyoko started, "don't feel pressured to drive in a way that doesn't suit you. Drive within your limit... take your time".
Yes! Take your time. There's no rush. No medals for highway riding. And the anxiety is MUCH better than overconfidence. Anxiety can be overcome with exposure, overconfidence can get you killed. Honestly, at first, I was pretty nervous on the highways too. Especially when the wind kicked up, which it usually does here in the afternoons. You will get used to it, I promise.
1. I tend to ride in the middle of the lane more in higher crosswinds. That gives me a few feet in either direction if a sudden gust moves me around. Obviously that should change depending on riding conditions, like rain, etc.
2. You can lessen the amount of wind force by changing your body position. Bring your elbows and knees in. Tuck your head down if you want. related to this is your clothes. Zip that jacket up before you ride. No need to ride with a sail attached to your chest. Keep that visor down on your helmet too. You're more streamlined, and you need to protect your eyes anyway. You can also play with tipping your helmet a little to change the way the wind affects your head.
3. Relax. I know, easier said than done. You have a few feet either way to move in your lane. A death grip on the bars is pretty much always a bad thing. If the wind pushes you over a bit, easy countersteering and lean will move you back. Trying to manhandle the bars will make it worse. Too many minutes of this kind of riding will wear you out too. Not good to ride exhausted.
Remember this!!!! At highway speeds, the gyroscopic force of your spinning wheels will keep you upright. Have you ever watched a video of a racer who falls off his bike and the bike keeps rolling along without him? Your bike wants to do that too. And the faster you go, the higher the force.
Last bit. My wife is still learning. When we ride together, and I feel like she's is tense on the bike, I tell her to flap her elbows like a chicken. Here's what I mean. With your hands still on the grips, move your elbows up and down a bit. This does a couple of things.
1. It forces you to relax your grip a bit because you can't grip tightly and move your arms around. It's like deep breathing in that it's a distraction.
2. It's actually practice in how small adjustments affect your bike and how the wind hits you.
3. It's kinda funny. You can't help but feel a little silly flapping your elbows.
You got this, YGodzilla.