The late producer Ismail Merchant (A ROOM WITH A VIEW, HOWARD'S END) once said that we tend to lump all forms of success in one grab-all bag. So a film's commercial success is often the barometer used to determine its success as art. And Mr. Merchant warned against these easy lumping together of things that do not necessarily belong together.
I cannot agree more. A film that has top numbers in the box office charts is not necessarily a good film. A festival hit does not translate to money at the box office--and sometimes, does not even mean a film is a good film, leave alone a great one! Critics rave about works that fail to meet the test of time, and time pushes to the surface works that were deemed commercial or critical failures...The list is endless. But time is a fair arbiter of these things, and in him/her, I trust. (Let's not forget that the mighty Hitchcock never won an Academy award for Best Director. But PSYCHO, and REAR WINDOW, and VERTIGO, and THE MAN WHO KNEW TOO MUCH are and will be evergreen.)
It's crucial for not only filmmakers, but for everyone with a project, a mission, a goal, to define what success means, for themselves. Do you want your work to reach millions? Do you want it to make millions? Are you looking for a calling card, to fuel your next deal? Do you want to win over the critics? Or get into a festival/festivals? Is it pure self-expression--with the belief that your best thoughts, expressed to the best of your skills and abilities, will in turn spark a reciprocal response in your ideal viewer?
Have a clear idea of what it is you hope to gain, by doing your work. If you are not clear on what constitutes success to you, it will be defined for you by other forces, by default. And you may not like what you end up with.