this is about amy's sister bonnie who died at 3, when amy was 6. she's saying that she's tired of living on her own when her sister can't.

I hate me,
For breathing without you. - she can't stand the fact that she's alive and her sister isn't
I don't want to feel anymore for you. - i dont think this means that she doesn't want to grieve for her sister anymore, it means 'i don't want to be feeling life on behalf of yoiu, you should be able to feel for yourself

Grieving for you,
I'm not grieving for you. - maybe coz she feels her sister is still with her, but more likely she's saying that she doesn't feel like she's grieving coz that means that you believe their gone whereas she wants to see her sister again
And though I may have lost my way,
All paths lead straight to you. - she's realised that everything wrong in her life, and everything she feels is because of how much she misses her sister

I long to be like you,
Lie cold in the ground like you. - this doesn't mean that she wants to die, but just that she know dying is the only way to see her sister again and so she wants to join her

Halo,
Blinding wall between us. - heaven i assume

Humming, haunted somewhere out there.
I believe our love can see us through in death. - she believes that because of how much the sister's love each other they can 'overcome any obstacles', even death

I long to be like you,
Lie cold in the ground like you.
There's room inside for two and I'm not grieving for you,
I'm coming for you. - theres a way for amy to join her sister, and she's grieving, so whatever it is she'll take it. she's coming

You're not alone,
No matter what they told you, you're not alone.
I'll be right beside you forevermore. - we're not parted even though you're dead and i'm not

I long to be like you, sis,
Lie cold in the ground like you did. - the past tense suggests that she knows that her sister is gone to heaven and is beyond amy's reach, but amy's going coming after her anyway
There's room inside for two and I'm not grieving for you.

And as we lay in silent bliss, - her sisters silent but amy feels she's there
I know you remember me. - they're still together

I long to be like you,
Lie cold in the ground like you.
There's room inside for two and I'm not grieving for you,
I'm coming for you. - like before

this is a beautiful song...