This is exactly what happens to me. I’m totally comfortable till I begin exceeding what I mentally estimate as 18-20 feet. It’s like I take one more bite climbing and my mind starts telling me to stop. Like you said I suddenly am super aware of being elevated off the ground and every ounce of my is uncomfortable. Sometimes I push through it to gain a few more feet depending on the situation. I’m normally fine sitting still at the height, but then when I stand to begin descending (especially in the evening) it hits me again till I’m below that 18 foot or so mark. Weird