In PC2 try playing with the world movement slider. One end of the slider (0 I think) makes the camera stay locked to the horizon and the other makes it locked to the car. Moving it should help depending on whether your brain is hitching to the horizon or the cockpit. It affects people differently so you'll need to experiment.
Assetto and Dirt Rally have similar options.
With it locked more to the cockpit you might also need to adjust the gforce slider or it'll be hard to see where you're going over bumps and curbs. Also make sure you're getting 45fps minimum, preferably 90. Any less and you'll start seeing display lag/choppiness which won't help.
It will get better if you do a little bit every day or 2. When I started I had to close my eyes whenever launching or stopping the car as that triggered me the most. You're brain expects g-forces that aren't there and thinks it's hallucinating so it goes into purge poison mode. Crashes aren't great either
Whatever you do, don't try to plow through it. You'll just train your brain to associate vr with sickness. If you start to feel queasy or you're getting cold sweats, stop for the day.