Her

Initial critique: punctuation. Why not use it?

What does she want?
Who does she want to be?
What has she become?

Her mind wanders.
Her body takes the pain.
Darkness emerges, and she asks,
"Was it worth the pain?"

My take on this piece: the speaker is narrating the general thoughts of a young woman while she is in the act of self-harm. That could be totally wrong, but here's my minor revision based on that view:

Her mind wanders.
What has she become?

Who does she want to be?
What does she want?

Her body takes the pain,
Is it worth the pain?

I would try to lengthen this poem by adding sensory details (colors, textures, sounds, etc.) so the reader has more to latch onto. Think about the space the subject occupies: is it inside or outside, describe the light source, create analogies for ambient sounds or physical sensations. The way we perceive the world and spell it out in writing is what makes our poems stand out.

You're using the same foundation that many writers have before (pain/darkness/who am I? motifs), and that's fine, but build your own house upon it. Keep writing!

I'm not a fan of poems that ask rhetorical questions and never offer an answer.

If you ask your reader:

What does she want?

Be prepared to give the answer to that question somewhere in the text of the poem. We don't know who "she" is. We don't care. We don't know what she wants. We don't care. We don't know anything at all about this person at this point, other than she's a she. That's surprisingly little information contained in a full line of poetry across 21 characters and four words plus the title! You can do better than this. Start by telling us what "she" wants according to the narrator – or better yet, tell us why we should care that she would maybe want it. Show. Don't tell. Be specific and detailed. Poetry is made of such stuff.