Read that passage again. The intense fear was a distraction. While you do need determination to get past the fear, the actual issue is a sort of palpable strain on the body. She got through by building physical strength through walking around.
I found the page in question: http://flipside.keenspot.com/comic.php?i=2647 "the stress is what makes it so difficult to move in dark cell".
Maybe I missed the part where they mentioned a physical barrier. I guess that the kind of fear in phase 1 is something of the extreme, even impossible to imagine. It should probably have more to do with general fitness and heart condition than body type. Anyways, I don't think it's that.
I find it interesting that Thin Man's qualia theory appears to be a dead end.
Indeed, or maybe May hasn't asked the right questions yet. It's all too freaky still.
A few things.
Basically, I think I would put it like this. It's basically like agoraphobia. When you go outside with such a condition after being a shut-in for months/years, the stress creates a physical sensation (high blood pressure, actual muscle strain, etc). But now that I see the other comments, I have a different theory about everything.
The qualia aren't something known to the two inside. This tells me Eye, at least, is innocent. And possibly that that term was something coined by Lehm and not their proper name. If she showed the painting CS3, she would like be like "Oh yeah, that's a..."
Back to the barrier. I suspect that not everyone enters the Dark Cell the same way. As in, probably when the original carrier dies, the Dark Cell opens up somewhere and random people enter, only to become the new host. The space closes around them, giving them despair, which they can't overcome, and they are trapped inside the cell with CS3. They can't of course leave unless someone switches off with them.
Which brings us to our side note. Cybele Surrogate Three. I suspect, surrogate as in, a replacement for the original Cybele. If I had to guess, I would say that Cybele (whatever her full name is) is likely the creator of the qualia, and there are three (at least) synthetic lifeforms made as substitutes (surrogates) to house her knowledge set and act as company for the one trapped in the Dark Cell. The interesting part is, CS3 is probably also jailer. She gave her back her eyes, hoping that it will trap her. So yeah, more likely Phase 3.
So basically, it isn't so much that she is trapped so much as the black aura surrounding her makes it unthinkable for her to leave (she'd be taking a huge zone of negativity with her. Usually, that makes it a "stay until you die, or find someone worthy to replace you" issue. However, all of this changes if May is able to somehow suppress or subdue the negative energy.