I've found a fear of heights isn't really a factor.
Sure, I feel queasy when looking at some YouTube video of some idiot on a boom-arm of a crane mounted to a skyscraper, or someone doing backflips about 2 inches from an edge that ends with a sidewalk a fair amount of distance below, but just being on a skyscraper or something doesn't do much for me in terms of feeling queasy or unsure of myself. I've stood on the Empire State Building, and could readily enjoy the view without feeling too problematic about it. Sure, looking straight down might still evoke feelings of dread, but there's not that much opportunity to do so on a skyscaper.
A plane gives me the same feeling. Sure, it's noticable that you're leaving the ground upon landing and departing, but the mid-piece of the flight you either see clouds, might see some ground (rarely enough to make out details) or you're over the water. At 30k ft it all looks so far away with so little detail, you might just aswell forget you're on a plane... You can't look straight down, and even if you do, there's nothing down there allowing for a sense of scale to indicate how high up you are.
As to forgetting about being on a plane... well, other than the too small chairs, the kids, possibly crying babies, the awkward position if you want to sleep, the dry mouth due to the AC and climate control going on, and the food that (besides being 'not the best' tasting - which is also partly due to the climate control/pressure/altitude apparently) you have to eat with your arms tucked in (to not bother any people in the seats next to you) and have little choice over what you get served (usually only 2 choices).
I remember a poster of a flight in the 60's with some people sitting around a table, in broad seats drinking champagne or something, captioned: 'Could you imagine where we'd be in 50 years?'
https://karryon.com.au/lifestyle/what-i ... s-and-60s/Granted, it wasn't that great in the 60s either going from that article, but at the least you're not crammed into a plane like nowadays. Well, I'm crammed in. I tend to need to figure out what to do with my legs as normally sitting would put my knees in the seat in front of me, so I need to stretch them out under the seat in front of me. I'm fairly broad so I always have to conciously tuck in my elbows to prevent poking my neighbor, and I constantly have issues with the armrest (which holds the remote for the on board entertainment, the buttons for the reclining, reading light and whatnot) on account of the seat being _just_ wide enough to accomodate my @$$. So my legs are always up against the armrests. Yeah, I know I could possibly stand to loose some weight, but everything is just about the bare minimum of what I'd need. Only way to make my flight more comfy, is to upgrade from economy class to business class or something. But financially that's usually a decent gap... which is why they keep calling it economy I guess.