Thanks everyone, this has been really helpful. It's so lovely to hear that other people have experienced the same thing, it makes me much less alone. The advice about diet is probably bang on; I've found that my appetite has completely disappeared so I tend to not eat for long periods of time then have a great big meal when I remember! Oops. I'm definitely going to try to look after myself more. On the thesis side, I'm going to nail the results chapter that I've been dragging my heels on this weekend, and crack straight on with the next one. It 'doesn't have to be perfect', is quite right. It just needs to be 'done'!
Thanks again everyone!

I have to hand my in my thesis in a mere 6 weeks (deep breaths now), and I am, quite frankly, freaking the hell out. I've got an entire results chapter to write, another one to finish, two more that need major corrections after my supervisors ripped them to shreds and the discussion to write. The only thing in the bag is the introduction. From this angle, facing the prospect of another long and daunting evening of work before dragging myself into my part-time job tomorrow, bleary-eyed and irritable, it all seems rather hopeless. I am normally a pretty stable person (at least, I like to think so), but I think I had an actual bona fide panic attack at about 3.30 this morning. Think sitting bolt upright in bed and hyper-ventilating for about 5 minutes, pretty scary. I've calmed down today and told myself that my thesis doesn't define me, that it'll be submitted in a few more weeks and I can start getting my life on track etc. etc. and am feeling a bit more positive, but am quite rattled by the experience. The reason for this post is therefore twofold: 1) I want to reassure anyone else nearing the end that a little bit of mental anguish is par for the course, and that it does pass, and 2) I shamelessly just want some cyber-PhD-folk to tell me that everything is going to be ok, and to hear your suggestions for talking yourselves off of the ceiling when the going gets tough (or, in my case, when the going gets overtly mathematical!).

If you are having doubts at this stage they will only become magnified if you accept this project. The fact that your supervisor changed the topic at such short notice is worrying too. I hear alarm bells ringing. I'm limping towards the end of my 4th year now and my advice is this: a PhD is spectacularly hard work, an all-consuming endeavour that will make you question everything you ever held true - not just about your subject area but about yourself too - and you should only undertake a project that you will really get your teeth into. If you are having doubts already then do yourself a favour and get out while you still can!

Making a thesis plan has undoubtedly been the most useful thing I've done in terms of writing up. It's rather, er... organic, and subject to change at a moments notice, but has been a sound basis for all my chapters thus far. I'm writing a science thesis, and I organised it from the middle outwards; 4 structured 'results' chapters, an analytical techniques chapter; an introduction and a discussion. I found it useful to make a master folder, 'PhD thesis' or something equally imaginative, then made 7 folders within that. Into each folder I moved all the relevant electronic papers and absolutely anything I'd ever written and any diagrams I'd made that were even vaguely related. By the end of an afternoon I had a (very skeletal) outline of a thesis! Try it, it really works, and is a great confidence booster.