It's the kind of poem that makes a reader wonder, particularly that first line:

The one with no face is the thing I fear.

Immediately I thought of something internal, as emotions of some sort. Maybe regrets, fears, anticipation, the lack of confidence. it could b so many things.

These lines of course could be a clue:

I hide inside my comfort shell.
Waiting for this thing to give me hell.

So the speaker hides in his comfort zone, knowing some thing is coming, like anxiety. That is a possibility. But it is going to give him hell, get him out of that place of comfort, so it is whatever forces him to be more outgoing, less complacent, that takes him out of that comfort zone.

These lines seem to throw that theory off, however:

I watch the clock and wait for it to arrive,
But when I look up, it has already passed me by.

It's about waiting, and yet it seems to come and go without being noticed. So maybe the thing he fears is timing. We all need the right time to do something, without it, we sit and miss our opportunities. So maybe that is it.

A you can see, I have no idea, though I've certainly made enough suppositions. Still, that is the fun of it, trying to see what the reader intended. Sometimes I'm lucky and the author tells me, other times, the writer has no one idea in his/her mind. But it's always fun trying to come to my own assessment.