I experienced the same when i was doing some of my exams 2 years ago (Met, GNAV & flight planning), i was all ready to pull out and then a bristol instructor convinced me i should have a go anyway so i did - 100%, 100%, 99% - don't know what i was worrying about in the end.
My point being you feel unready but somewhere inside you the information during the brush ups sinks in and bubbles to the surface. My advice go and have a go.