Hi alt
My first flight over 6K gave me the creeps, so much, that on the way back I took a major detour flying valleys. Since then, that feeling most of the times disappeared and I am been flying regularly over 10K in robbies, even prefer it over being shaken by lower valley or ridge winds.
I think it is a combination of
(1) fear of heights
(2) the fear produced by a feeling of imbalance.
The first part can be softened by flying "shouldered": keep a mountain ridge just above your "shoulder". I think it can be trained by going progressively higher.
The second part comes from loss of reference. This is consistent with the testimonies of progressive fear at high hover, or when there is turbulence, or over sea. For this part it is useful to be comfortable to be able to fly "IFR", that is without visual reference. At high alt I have "one eye on the instruments" because turbulence and updrafts will provoke unbalances (speed, climb rates, vibrations) that are not easily controlled because reduced outside visual references. This is more pronounced in light helis without stability augmentation nor autopilot.
m2c,
d3