Wonderful text. I must remark that even without the "oomph" it's well written. Good job.
The way I revise is to read through the text and mark spots where my mind halted and made me rethink the sentence.
"I didn't feel anything at first."
Instead of feeling nothing, I think we always feel something. Take yourself as an example. When you are standing at the bus stop and feel "nothing". What do you actually feel?
I know I feel uncomfortable standing, or I get an itch at my back or anything else that pulls at my attention. There is always something we feel. It might just be small enough to get unnoticed in usual day to day life.
So instead of saying she doesn't feel anything at first, try to focus on what she is feeling right now. Maybe it's the after taste of the potion, or the way that the bottle feels in her hand, it can be the sound coming from outside that pulls at her senses. Anything and everything she feels. It's also to build a tempo towards the pain.
"...because this...this was pain. Pure raw pain."
Nice of you to tell us this, but what is this pure raw pain. You could argue it's for a reader to interpret what it actually feels like, but why waste an opportunity.
Instead, give us the pain, tell us how she howls and thrashes on the floor, how her uncle's hands hold her down to the floor, how her head smashes against the cold stone. What else is she sensing, can she taste blood in her mouth? Did she bit her tounge while thrashing around?
Make us, readers, never want to taste this potion she drank.
"But I couldn't do anything. I curled up into a safe space in my mind,
where it was comfortable and cold. Here was safe. I could be here. it
was safe here."
I had difficulties getting the picture you wanted to paint here. You said she passes out, and even though I can see it, if not for an explanation I would have had a halt moment where I had to read the sentence again.
Again focus on her senses. Her vision could be darkening, she could be feeling the cold of the floor beneath her, make her shiver because of it, despite the flush of heat from the potion. One thing you already mentioned is her faintly hearing someone call her name. That's great. Just give us more of it.
Another thing is the "I was safe, I was safe, I was safe." Feels like she is still present while saying those things.
It makes it seem like the pain passed and she is passing out because of an exhaustion. Instead, I think the cut needs to be instant, the pain pushed her over the edge and she feels nothing, can see nothing, hears nothing.
Reading the text once more I think you just need more showing less telling. When I began writing an answer I also thought you could improve the tempo, but it's fine.
Upon reading the answer from 21st-century writer I can see the sentences are a bit short at moments when extending the pain and the things she feels would have made the tempo better.