Ok, I'm not sure what The Used was thinking when writing this song, but it comes across to me as...

'He wears his heart safety pinned to his backpack
his backpack is all that he knows'

I think what it's saying is he guards his heart, his backpack doesn't need to necessarily mean a literal backpack, but lets say it does, he keeps his backpack close, it's all that he knows. So he guards his heart on his backpack so that no on could hurt him or whatever.

'Shot down by strangers who's glances can criple
the heart and devour the soul.'

Well we all know those people who judge us by our outward appearances, the way we show ourselves, what little experience they have had with us, etc. Basically, there are strangers who watch him in disgust because he is 'different' then them, but their glances are painful and hurt him.

'All alone he turns to stone
while holding his breath half to death'

This varies, the first part comes across to me as when he is alone, all happiness leaves him, he becomes depressed, and nothing can chear him up--turning him to stone. Obviously, stone cannot feel, so it may also be a way of numbing himself to the 'glances' and hurtful words, or whatever other people might do to him. The 'holding his breath half to death' is a little more difficult. A lot of people in depression are delusional too. Like they have schizophrenia. Sometimes holding your breath and counting slowly to ten, or higher, may clear your head of these delusions or whatever.

'Terrified of what's inside
to save his life
he crawls like a worm from a bird'

I think this is pretty straight forward, but maybe not. When in depression, the feelings come from inside, they creep around telling you what's wrong with you, basically giving you your depression. He is terrified of these thoughts, and like a worm cowards from a bird (for those of you who are a little slow...it's because birds eat worms.) he cowards from these thoughts.

'Whispers of...(just keep working...just keep working...im terrified)'

Just keep working, to keep the thoughts off of his fears and why he is terrified.

'Out of his mind away
pushes him whispering
"Musta been out of his mind..."

I'm not exactly sure what he means here, probably because it's slightly hard (to me) to understand the way it is written, but if it means people are whispering that he's out of his mind, then he is trying to push away those thoughts and what those people have said.

'Mid-day dillusions of pushing this
out of his head
make him out of his mind...'

He wishes (delusions that he can), but is unable to push these thoughts out of his head, no matter how hard he tries, and it makes him go out of his mind.

For the next few parts, it is pretty much the same as I have already explained...

This song is a little confusing, and I might even be wrong in my interpretation, but I hope this helps anyway :).