When I first started VR I would get sick, so I started to use it for 30 minutes at a time, then 1 hour, then more and more until I can handle 8 hours at a time (break at 4 hour mark). It just takes practice.
did you grow up playing a lot of videogames? I have a theory that getting used to it while young has a big impact on how it'll effect you later. My grandma can't even look at Minecraft without getting sick and I can play VR for hours with nothing other than a bit of "woah no delay" for a few minutes
Curious if you started with 'intense' experiences with artificial locomotion? IIRC if you have bad experiences while getting used to VR, your brain will associate the headset with nausea and it can be a vicious cycle.
Some years ago I got so drunk that had a 1-2 seconds visual delay, I vividly remember not being able to stop my friends from stealing my chips because I literally tried stopping them way too late. It was really funny tbh, damn.
In any case, I never felt nauseated, my brain got accustomed to having a delay naturally.
If you think about it, the perception of what is "right side up" is arbitrary. It's like that cliche discussion of qualia and what if what I see as red is different from what you see as red.
The lenses of your eyes already perceive the whole world upside down, your brain just flips it. If you wear glasses that invert it after a few days your brain will stop the flipping (and then if you take them off it'll take a few days to re-flip).