Im a young'un who has ridden BMX for the best part of 15 years.
I generally ride what i consider very much to be in my comfort zone (which would easily encompass a 4ft drop or 6ft double) and get very disappointed with myself when i bottle out of something i consider should be in my comfort zone.
I went to woburn and bottled the BIG double the 30ft+ one as well as the big drop from the top into the jump area, i got pretty bummed out.
For me, it's the first pedal stroke, as soon as that is done, i'm normally 99% composed and do it easy. It's the whole nervousness and build up before the first pedal stroke that gets me.
Also, big, long, featureless run-up into something massive are so difficult for me, i have no idea on speed. It's from riding BMX trails all my life, there are generally features that dictate your speed rather than guessing.
I also cant follow MTBers/DHers into jumps, they go way way faster than me to clear the same jump, i just end up going flat at their speed. BMXer pump.