I’m not usually the one to post things or asking but I am struggling at the moment, especially knowing I’ve got my Theory Test tomorrow that I’ve not revised for!
I know this isn't much help, but honestly the absolute worst thing that can happen is you fail. And so what? I know it's crap to fail, and in your situation I would probably feel the same, but honestly it wont be that bad if you don't pass. It just gives you more experience of the test questions and then you can rebook and have plenty of time to go over stuff for nex time.
If you cancel it you definitely 100% wont pass tomorrow, that's just a guaranteed :P But if you get a good nights sleep and just do your best you might just pass