My Chemical Romance - Honey, This Mirror Isn't Big Enough for the Two of Us Lyrics Meaning

Okay, since no-one wants to break this song down verse-for-verse, I guess I will.

The amount of pills I'm taking
Counteracts the booze I'm drinking,
And this vanity I'm breaking
Lets me live my life like this

This part is something that a lot of people in rock/punk/metal bands seem to think: I'm indestructable because I make music. So Gerard thinks alcohol and drugs won't affect him "lets me live my life like this"

And well I find it hard to stay
With the words you say.

Either a friend or romantic partner are intervening and Gerard doesn't like it/care, so he's thinking of cutting them out of his life.

Well I'll choose the life I've taken,
Never mind the friends I'm making,

He's decided that this is how he's supposed to be and that he doesn't care that the people he hangs around are a bad influence on his drug habits

And you can cry all you want to, I don't care how much
You'll invest yourself in me.
We're not working out.

At this point, the person who is trying to talk him out of it is visually upset and Gerard's blowing them off. In the case of that person being his girlfriend, he's breaking things off because she doesn't want him to continue usuing drugs.

And you can't touch my brother,
And you can't keep my friends,

The girl has now informed his friends and family of Gerard's addiction. He finds out it was her and tells her off; not to do it again

Never mind the times I've seen it.

He's seen other people get hurt and killed because of drugs but he doesn't care. He still thinks he's 'invincible'.

Well I hope I'm not mistaken
By the news I heard from waking.
And it's hard to say I'm shaken
By the choices that I make.

He's OD'd and passed out, when he wakes up he's told and he doesn't care. It's most likely not the first time it happened.

Well I'll choose this life I've taken,
Never mind the friends I'm making.
And I get a little shaken,
Because I live my life like this.

At this point, if you listen to the song, his voice gets rougher and more panicked, indicating the drugs are affecting his sanity. It's also in the lyrics: "And I get a little shaken because I live my life like this" I also think at this point the girlfriend has moved on, giving up on Gerard altogether.

And you can't keep my brother,
And you won't fuck my friends.

The girlfriend has resurfaced, now with his family on her side. It appears she's dating one of his friends/band mates "You won't fuck my friends"

Never again, never again.
(You can cry all you want to I don't care how much you'll invest yourself in me.)
Oh baby let me in,
I'm knocking let me in.

He's now outside her door, regretting his decision to not listen to her. He's hearing all the things he said to her in his head."(You can cry all you want to I don't care how much you'll invest yourself in me.)" And she's not listening because she moved on.