I don't know what type of phobia it is, so I'm sorry I can't help you there, but I totally get your struggle! My fears aren't as bad as a phobia but I suffer with a lot of anxiety over various topics, including this — I've always found it's because of how close the ground is? It looks like an optical illusion, for one, and also, it may be because you know (subconsciously or not) that if you fall from a lower hight it'll hurt like hell and not kill you, whereas a higher hight is more likely to do the latter. That's what it's been for me in the past. Hope this helps a little? Either way, sorry you're struggling with a phobia. They suck! Well wishes : )