I have my viva in three weeks. I've prepared quite a bit, but I find that on reading my thesis I think it's actually very weak, poorly argued, and I'm really not sure I can defend it. I was very poorly supervised, with very little of my written work ever inspected (I ended up submitting the thesis without it even having been read by anyone, for reasons of time pressure), and am now deeply concerned that I will fail, which would be a devastating experience.