What am i?

I'm the shiver in the night
that you thought was from the cold
The voice in your mind
repeating the story once told

I'm the nervous breakdown
you almost had
When HE didn't want you
I thought you'd be glad

I'm the smile in the mirror
that just doesn't seem right
Twisted like a upward frown
Quite an ugly sight

I'm the black stained pillow
Caused by all your tears
Bleeding your mascara
since it's the only way you can

I'm the razor hidden carefully
One of three
Two of them together
and then the loner which is me

I'm the sharpest and the shiniest
The least used but most liked
I know because the last time you used me
Everything felt so right..