I think that Missing is about Amy making a mistake in her life and even though she's apologised and accepted responsibility, no one would forgive her.
"Please, please forgive me"
self-explanatory
"but i won't be home again"
she feels as if her home wherever she feels safe has been taken away and even though she tries; Amy cannot get it back
"maybe someday you'll look up and barely concious, you'll say to no one"
she's saying maybe someday the people that ignored will make a mistake like Amy did and they'll know how she felt.
"you won't cry for my absence i know
you forgot me long ago"
Amy is speaking to her boyfriend or lover and basically saying that he won't feel sadness when she's not around. second line is pretty obvious.
"am i that uninmportant?
am i so insignificant?"
also self-explanatory.
"isn't something missing?
isnt someone missing me?"
shes speaking about how in her absence shes not missed or thought about.
"even though im the sacrifice
you won't try for me not now"
even though she was the one to make that mistake therefore the "sacrifice" none would stand up for her or listen to her side of her story.
"though i'd die to know you love me
i'm all alone
isn't someone missing me"
she'd give up her life for her b/f to love her again, but she's all alone.
"i know what you do to yourself"
Amy's singing about how she tortures herself with the thoughts of no one caring or loving her.
"and if i bleed, i'll bleed
knowing that you don't care"
if she becomes mentally or physically ill, her lover won't care about it.
"and if i sleep, just to dream of you,
i'll wake without you there."
when she sleeps, she sleeps just to dream of the memories she had of him but when she wakes up he won't be there.
DONE!!! if i missed out some lines its because i already interpreted them or they are are so self-explanatory that there's no point in explaining them
this is just my interpretation and i apologise if its wrong or doesnt agree with your opinion but its what i think the meaning is basically :) :)
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