Ok, take a breather. Yes, absolutely look at the feedback.
If it's due in two days, you are nearly there. It's not rubbish, you know. It just won't be. You have been getting marks between a first and a 2:1 through your whole course, so there's no way this is rubbish. Put the panic out of your mind.
Now, could it be better? Perhaps. Think about that. What can you do quickly and calmly to make it better?
Good editing is a big thing. I mark dissertations regularly, and people come down on issues like referencing and proofreading all the time. An essay scoring 63 might be just as sophisticated and interesting as one scoring 66, but the one scoring 63 has lost points - and slipped from a mid-high 2:1 to a low 2:1 - because the referencing wasn't there and it was badly proofed.
So, if you are nervous, sit down and do some simple, thorough work on the bibliography and footnotes, and make sure everything conforms to the requirements. This will also give you breathing space.
Next: why are you so worried about this piece? Ok, sure, everyone thinks they could do better. But you have done well in your course so far, and this piece of work is likely to be just fine. If you feel bad about it, it may be that you're having the normal crisis of confidence we all have about important work. Or, it might be that you're aware you've not put your argument across as well as you'd like. Either way, here's what you need to do:
Look very carefully at the introductory and concluding paragraphs. Are they clear and punchy? Do you instantly know what the argument of the dissertation is (at the start)? Do you know the important take-away points (at the end)? If you are unsure, look at a journal in your field, and look how people write their first and last paragraphs. Model your work on these. A clear introduction and conclusion could pick you up important marks.
You don't likely have time to do more, unless you have actually not finished writing/drafting! Take this on, do it, and then if you have time over, do whatever revisions you want. But keep calm.