Jeanne's edit

Jeanne--
set up: she's looking at her reflection in the mirror and she is covered in blood, but only in the reflection

It dripped from her hair, weighing it down until it resembled a weeping willow. There was a jagged crimson line separating her head from her body. Blood trickled down from it forming red rivulets that merged with the innumerable cuts marring the porcelain perfection of her body. Some were long thick gashes created by hands hyped on the adrenaline of battle. Some where thin straight lines crafted with surgical precision. Some were symbols whose meanings had long since been lost.


It dripped from her hair, weighing it down until it resembled a weeping willow.

I thnk your difficult task here is to make clear (without being too obvious) that she's looking in a mirror, and the mirror image is different than reality. So let's be in her. She's looking in the mirror and sees blood. (You might have this already, but carry it through.)

You might have done this purposefully, or maybe you're just resonating to the centrality of the mirror motif in amplifying and contradicting the notion of identity and self. The "looking in the mirror" exercise is used, you know, to test consciousness-- that is, when does a baby realize that it's her image in the mirror-- it's not another real baby? (It's very amusing to do this with a cat-- he'll try to get behind the mirror to get at the other cat. Of course, being a cat, once he realizes he can't figure this out, he ignores the mirror in his lordly fashion.) The reversal of the image is also important. How long before we automatically scrub the smudge off our right cheek when we see the smudge on the left cheek in the mirror?

Anyway, as soon as we incorporate the idea that this is a reflection of us, we react by taking in the information about us. So if she's seeing her own image there, what's the first thing she's going to do? I think she's going to put her hand to her head, to make sure that it's not really bleeding. Then she might look at her hand-- no blood. Then back at the mirror, and then she would catalog the rest of what she sees.
It dripped from her hair, weighing it down until it resembled a weeping willow.

Watch your "it"-- use the noun when you can-- you've got three "its" here, and two of them are about hair, I presume, and one about blood. Use the noun:

The blood dripped from her hair, weighing it down until it resembled a weeping willow.

That "until it resembled" feels more clunky and "written" than the rest of the passage. How about:
The blood dripped from her hair, weighing it down like a weeping willow.
or till her head looked like a weeping willow? A bloody weeping willow? A willow is green, so you might say "red weeping willow" so we don't get the wrong idea!
There was a jagged crimson line separating her head from her body.

Where was the line? Across her throat? Again, she might touch her throat (and does her hand in the reflection go to its neck too?) and look at her hand again. Be in her body, and see through her eyes. What does her body do? She's not just a seeing machine-- her body is going to move instinctively.
Blood trickled down from it forming red rivulets that merged with the innumerable cuts marring the porcelain perfection of her body.

If she's naked, say it-- of her naked body.
Some were long thick gashes created by hands hyped on the adrenaline of battle.
Some what? Some of the cuts? Don't worry about being too long here-- length will help, I think, because you want this to be important.
Now "hands hyped on the adrenaline of battle"-- does she know that? Is she the one observing that? It seems sort of omniscient.

Some where thin straight lines crafted with surgical precision.
Proofread. :)

Some were symbols whose meanings had long since been lost.

Are you in her viewpoint? If so, does she know the meanings had been lost, or does she just not know what they mean?

I would suggest ending with something that is hers-- her thought or her movement. Don't lose her here. Now you might be going for something different-- maybe this is a common experience for her, to look in the mirror and see the future, maybe? No matter what, though. I think the sight of blood on your head and slashes in your body would be primal enough that it would be really hard not to react somehow. If she doesn't respond, make some point of that, that she's clinical, maybe?

Great idea!
Alicia