My opinion is fairly worthless, but I'll give it to you. First, know that I almost always favor taking out words and phrases that aren't doing much, so here's the minimum I would leave in the poem:
Blood seeps from my veins
all feelings carried with it
a river of emotion
running down my arm
I feel almost pain
I slice again
Slit shaped holes in my arms
across my chest, my face
They are leaking, dear,
I am drenched in my soul
How does it feel to be imprisoned
within this body; within these veins
With this blade I free myself
I no longer feel in vain
The end of the second stanza just had too many "ands" for me, and the phrase "I feel so good" just seems overused and doesn't really convey much. Perhaps you could keep whole line if you replaced it with something more descriptive, like "I feel X" or "I am X", where X is some juicy word.
For the third stanza, once again I dropped an "and". I like the sss sounds in the stanza, perhaps some word swapping can bring it back in for the last line. Especially since I don't feel like "drenched" is working. I do, however, like the image of the soul and the blood being one and the same. Perhaps you could incorporate that elsewhere in the poem.
Fourth stanza: the question doesn't really make sense. You introduce another person in the third stanza by referring to "dear" and then ask them about your
body. Are you talking to yourself? If so, make it clearer. If not, who is this person? Readers get distracted by weird inconsistencies like that. (At least, I did.) I think that being "imprisoned // within these veins" is more vivid than being "imprisoned // within this body" and less overused. It gives me the image of veins being ropes or chains or something, actually restraining you. Perhaps that could be developed more. And I don't really understand the last line.
Here's "my version", with lots of things removed.
But I still don't particularly like the last stanza. Perhaps remove it?
What they (mathematicians) define as interesting depends on their particular field of study; mathematical anaylsts find pain and extreme confusion interesting, whereas geometers are interested in beauty.