To be honest, if the weather get's really bad and stays that way, I just abandon the whole thing. I mean it's not fun is it?
There are few places in Europe, and none that I know of in England and Wales (not sure about Scotland) where you can't get the hell out and find a pub to while away the hours.
The last time I hit this problem was when we'd spent a day walking in rain ranging from torrential to drizzle, and then woke up next morning, facing an 18 mile mountain walk where you couldn't see a thing above 200m. So what was the point? 18 'beautiful' miles where all you could see was the rocky trail in front of you. A wonderful exercise in navigation, but fun?
As to the original question, the first night is always the worst when camping. I never sleep well, but after that like a baby. I've never had a mindset that thinks in terms of 'how many more days of this?'. I've always taken each day at at time, and just worried about the trials of the day. After all, if you don't make 'today', then there's not a lot of point thinking about tomorrow is there?
S.