Hi Paperplanes
I have had the same feeling while learning to fly and flying solo, i think going by my own experience the higher you go the less sense of movement there is ,which then gives you the feeling of standing on a tall building.
And of course you want to leave that situation but you can't which then causes anxiety and the claustophobia feeling.
At the time i was learning i was very stressed at work and suffering panic atks , i did manage to get my ppl but let it lapse after a few years mainly because of these feelings.
If i could go back in time i would have seeked help straight away because in the past 20 years i have missed some lovely flying experiences
So get some advice /help you rationalize your fears , but as i have learn't the only why is to face your fear to overcome it (with help !)
Good luck and keep going !