I think most people struggle with confidence issues at this stage, in particular thinking is this really good enough for a PhD? I always set over-ambitious deadlines (e.g. writing one chapter per week) and then get really fed up when it doesn't work out! Also you are pretty much working without supervision, but in my experience when you do get feedback it's much harsher than in the earlier stages, which doesn't help with the confidence issue!

Sue, I think 3 months is reasonable for a draft of a thesis. If you have done a lot of papers and/or have written a lot already I'm sure that is do-able. My head of dept brags about how he wrote up in about 12 weeks(I think), but he got money to write up within a time limit.
I've heard there is a tendency to try and perfect the thesis and you have to just give in, say "this is good enough" and hand in or you can spend ages nit-picking. It depends on the person and situation.

Mine took three months but not continously... I have submitted now and am waiting to defend in mid-june. In the mean while I am in Srilanka for a holiday en route to a conference in Hong Kong.

Life is good once you draw that line and submit.

I am sure it will be better once the viva is over.