Something I like to tell my students is that good music comes from a place of simplicity. If you are struggling to execute whatever musical ideas you are trying to express with your playing, your attention will be on the technical demands of what you are doing. There won’t be a lot of focus left for the music itself. The result will be playing that sounds technical and unimaginative. If however you play from a place of simplicity, a comfort zone well within your technical capacities, then the technical aspects of your playing can go on to auto-pilot, and you can give 100% of your attention to the musicality and expressiveness of what your are playing. The result will be much more musical and interesting for the listener.
You will also find that adopting this approach helps everything stay relaxed, so that you play with greater finesse and accuracy, even if it might be well below your technical capacity. I find that as one tries to push one’s limits technically, one often tenses up, which results in stiff and inaccurate playing. So play what you find simple, stay relaxed, and focus on the music rather than your technique when playing.
Of course as one develops as a player, through regular steady practice, one’s technical capacities increase. One’s zone of simplicity expands to encompass things that would have once seemed impossible. In fact, you can look at your practice routine as a way of expanding your zone of simplicity.
So when performing, play in your zone of simplicity, and concentrate on the music. And always be working on increasing your zone of simplicity through your practice routine. But don’t take my word for it, try it out in your own playing, and see if it works for you!