It's weird how it changes for the worse. I really didn't like heights all my life, but nothing that I couldn't handle with some deep breathing. Now, I just freeze. I completely get the phrase "paralyzed by fear". And it's even worse if it involves DS - for instance, we were on a look out in Pittsburgh and even though DH was far from the edge, when he put DS on his shoulders I FREAKED. I mean completely lost it. He looked at me like I was nuts, which in that moment I was.
Also - interestingly enough, I think it's genetic. And I don't think it's a learned behavior (regardless of the story above, I'll explain). My dad is extremely afraid of heights, and I didn't live with him or do anything at a young age with him where I would have learned to be afraid.
Prior to my total freak out in Pittsburgh, DS (at age 2-3) would freeze at the tops of slides and shake all over. He wouldn't even attempt to go into the indoor playgrounds because typically you need to climb up a jungle gym type slide. He would watch the other kids and be content playing with his feel firmly on the ground. The only way he got to the top of a slide is if he was too caught up in the moment, followed another child up, then he would realize where he was and I would have to go up and get him. Each time I let him try to see if he would be ok.
Now, he's much better with playground equipment, but he still doesn't like high stuff. Took us forever to get him on Splash, but he likes "fast" so he tried it, and loves it. He does get really nervous at the top, but who doesn't?