I thought 'sod it' on the day of my test also.... the problem was I was already 45 minutes into it and had very nearly wandered into the Solent class D. My route involved crossing it once and then turning back to go through the side of it again and we were refused a transit on the first attempt. This put me into a bit of a state and I then had my brain engaged in kicking myself rather than just getting on with the situation as it had presented itself.
I did carry on to complete the handling section and went on to pass the nav section on a different route a few days later.
After the first partial result I was debriefed and was told that my problem had been that I had taken off with only one plan. This was actually a valuable lesson. Always have a plan B! What I should have done was to have planned a diversion as soon as my first route became unavailable to take me through the zone by a different route. What I did was to continue on the planned route at a lower altitude which left me in all sorts of problems.
The test is not difficult. As everybody has said, if your instructor says you are ready then you are capable of passing. If you don't pass there is no shame in it and you can just do it, or part of it, again.