Music is always in flux, as we are
I would start by telling myself to broaden my horizons and be more organised. When I was young, my approach to learning music was quite narrow and now I get inspiration from many different sources. Recordings from different eras are now readily and cheaply available. Even ordering books from libraries for research about composers and the time in which they lived has become simpler. Through time and life pressures, I have learnt to become more organised and productive in my practice time.
I would also emphasise that even though we are working constantly to improve our skills as musicians, perfectionism is something best left in the practice room. Performing can be a more creative and freeing experience when we realise that a piece of music can never be performed or experienced in the same way twice (nor should it be). Music is always in flux, as we are. Sharing the music with an audience in a performance is always a privilege, and our own doubts and niggles about our personal shortcomings belong in the practice room, not on stage.