Does anxiety increase the likelihood of developing motion sickness?
It's different for everyone, but I'd expect stress/anxiety to be a contributing factor for many, which is an ultimately self-defeating cycle; if you "know" you're going to get sick, you'll be anxious about that, and that'll make you sick.

The trigger for me is inadequate ventilation. I'm fortunate enough to rarely get motion sickness, but when it does happen it's usually because I'm in a hot, still cabin without a good source of fresh air.