Pages of advice, but I have found your problem a common one amongst students nearing the completion of their PPL. Many times I had a student with the same problem as you. Most of the time, the reason was that the student would have his lap, the rear seat and any other flat surface of the aircraft covered with charts, plogs, frequency books etc. etc. This would be all very good preparation, but as the student was being trained not only to pass the exam, but also to fly solo in the real world it was a little unrealistic. I found that all this wonderful preparation would have the student like a one-armed paper hangar just trying to fill the boxes in, as a result the heading and altitude keeping were quite often abysmal. My solution for students, if this was the case, was to simply take all this wonderfully preparation navigation documentation and throw it in the rear of the aircraft, give him the map only, show him where he was and say OK take me to this point or to this airfield. Suddenly his head was out of the cockpit and looking at the ground and relating the map and terrain information. Also, their heading and altitude keeping improved.
Sorry it's so long winded, but get the eyes outside, pick a point to fly to maintain your heading and a place on the windscreen to place the horizon to maintain your altitude look at the world outside, relax and ENJOY it.