To be a musician you have to be in love with practicing. You have to love the logical process of breaking something down smaller and smaller into manageable segments, then painstakingly putting it back together. You have to love the process of treading the same ground over and over again, striving for perfection but being comfortable with never reaching it (because it doesn’t exist).
You also have to get comfortable with the difference between playing accurately and playing well. This is easy to lose sight of in our current musical climate, which values accuracy so highly that sometimes it is conflated with art. It is possible (indeed, required) to play with total accuracy; it is impossible to play with perfection. There is always something that could be better. Always.
You have to try to get inside the composer’s head. What did she want you to show the audience at this moment? What’s the most important thing, or the most interesting thing, about this passage, and how does it fit into the overall structure of the work? You have to work out how you will get the composer’s thesis across to the audience; the tools in your box are articulation, dynamic level (volume), slight variations in speed, and other tiny nuances. Your solution won’t be the same as any other musician’s and this infinite variety, not the technical sameness of mere accuracy, is what makes music interesting.
You have to get inherent reward from the process because practicing is hours and hours and hours of your life, much more time that you will ever spend performing or even rehearsing, and if you don’t love it you will lose your fucking mind.