Huh, me too -- down to identical behavior as you with buses, planes, and trains.

If any of us ever figure out why, I'd love to know. It can get very difficult at times where I always have to drive and have to consider buses hard.

I think it's because you know where the car is going to do when you are in the driver's seat, so you can mentally brace yourself and adjust your internal balance systems accordingly. Whereas, as a passenger, you have to react to the car instead of controlling it and so your internal balance systems are always slightly behind the actual motion, causing motion sickness. This is probably extendable to planes, trains, and busses.

The ‘treatment’ that accidentally worked for me was desensitization through VR.

Interesting. Do you mean playing a specific game or just using VR in general?

Not any specific game, but some have character motion that isn't strictly coupled to player motion (e.g. performed by a controller like conventional first-person games), and I found that very unpleasant at first. Gradually I got used to this disconnected motion; the only thing that still made me queasy was disconnected rotation, but that is rare (moving platforms in Doom 3 are the only example I remember).

I used to suffer motion sickness as a passenger in cars, sometimes buses, and ships, but at least the first two went away along with the ‘VR sickness’. (I haven't been on a ship since.)

Do you get sick on buses, trains, or planes?

Buses sometimes, trains and planes, not so much.