Hi BTH
OK, I work in this field and will attempt to provide some help. However, I do this with hesitation... anyway.
You need to determine the cause of this. One option is that it is a pure sensory association of anxiety or fear that you have made at some point. Mostly because of some kind of experience in your past. These sensory associations are purely associative in nature and has no rational base or explanation (well it can have but not necessarily).
Another option (or added to the above) is that as we getting older or having more experience in life we change our beliefs about lots of things, including flying.eg. we are more aware of the dangers, we value our lifes and family life differently, etc. So, you could potentially think of an aspect of flying as more dangerous.
There are more reasons, e.g. your sense of balance also needs your eyes for reference and the higher you fly the less your eyes can help your ears(balance).
What to do? If it is the sensory option, go get in a heli and fly at your fear height until the feeling gets slightly easier. Repeat this a few times. It is important to stay at that height until you sense a reduction in the feelings, even if it is very small. Sensory associations are not strong and can shift relatively easy. If you avoid it, you just reinforce it.
If it is the belief option , well then you need to do some reflection on those beliefs and either work on changing them or to counter them with alternatives.
Best is to go and see someone as this is not necessarily a big problem! Then it is easy to determine the cause and find a solution.
Good luck
MVNH