Confidence
Agree with a lot already said ref discussing with an instructor.I'll bet he/she has had a little crisis of confidence at some time.
If you plan meticulously,concentrate on your timings/headings then it will work.I am always filled with a little wonderment when the waypoints come up on time but it is a tried and trusted system.
Once you have calced your g/s,put 6 minute (1/10 of an hour) marks on you chart lines as it is easy to do the mental maths i.e.g/s 90knots put marks 9nm apart and mark them 6/12/18/24 etc.
Try to get a mental picture of your route i.e. first leg is roughly south,2nd leg sort of southwest etc to ensure you have not made a gross error.
If you have VOR/DME make sure you know how to operate it-all a/c I have flown always seem to have slightly different panel.You don't want to figuring out which is NAV1 or NAV2 whilst in flight as this will put extra pressure on you and distract you from holding your headings.
Good luck