The best writing advice I ever received was to approach writing as a profession and not as the endeavor of an amateur.
Professionals hone whatever skill they have to create a product that people will want badly enough to give some of their pay/time/effort/lives/etc in order to enjoy it. As they create, they think of other peoples’ wants and needs first and set aside their own wants and needs until they are finished.
Amateurs, on the other hand, think of themselves first, last, and only. They write to exorcise demons, become famous, dabble in the creative arts, stretch their minds, or for any of a thousand other reasons that have to do with themselves instead of others. Their reasons may be good ones and possibly necessary for them and may fulfill some inner need but they do not lead to the creation of the kind of professional work that belongs in the larger world.
It was good advice when I received it and it’s still good advice.