The world just screams and falls apart

It hadn't been reaching the top that terrified her. It was coming back down.

You may think you understand her fear, but it wasn't that at the peak life was fantastic,

No, unlike heroin, once you're up, you don't feel on top of the world.

It's more the fact that getting to the top was such a struggle.

In the beginning, it was fun. It was an adventure. It was exciting, in fact, everything felt so fabulous that she didn't even notice the negatives- perhaps in that way, it was a little like heroin.

But the further she went, the less often she experienced those rushes of excitement. The slower her pace. (But she was so far up by then, that pace didn't matter anymore.)
The more the negative aspects seemed to leap out at her, shake her, make the climb a little more draining. And they began to drown out the positives.

So when she did finally reach the top, she was comfortable.
There was no more struggling to get anywhere- at the top, everything was constant.
Everything was safe.

Yes, perhaps she'd be happier if she came down, but the journey would be difficult.
At least you're not looking down on the way up.
Coming down would mean risking falling.

She was perfectly safe from change at the summit.

And she did not want to fall.