Looking too much at your instruments instead of looking OUTSIDE where the other aircraft are. You can perfectly fly without any instrument if you know the "nose picture" for speed determination.
For safety and navigation you should look outside. This was my personal biggest thing after many FSX hours.
FSX is good in learning the basic 3 axis and instrument flying. While training for PPL you will learn the 3 axis in 10 minutes anyway and you shouldn't be using the instruments. Learn to fly without them because they can and will fail someday.