FNG makes a very good point, it's always so easy to convince yourself that you are at an altogether different point. A lot of it is to do with relaxation, but the who is relaxed on a checkride apart from maybe the examiner? You can only be aware and do your best to relax! Try to study the sectional and look for as many waypoints as possible whilst you are on the ground but don't look out for just one when you are in the air(fixate).
Speaking of fixating, that's probably the biggest problem that I have had to deal with. Fixating on any particular instrument causes everything else to go pearshaped. A good instrument scan is an essential part of airmanship especially important later on in your training.