Why shouldn't you? That's a very fair question. Or so our mind tends to think when we are in that place.
You want to feel something. Right? You want to let out the pain and hurt that is inside of you. Give it a way to escape from your body. But cutting will only release blood, not the real issue. Not the real pain and distress that is causing these feelings.

It's funny how sometimes we know all the reasons why we shouldn't. We know logically why it's a horrible idea. Yet the emotions and urges so badly want to overpower that. Hang onto the truth, my friend, not the feelings.
Hang in there. You will make it through.