As you become more skilled at production, as you learn new techniques and improve your listening skills it's easy to forget that if the song's not working on a musical level then, well, any extra effort spent mixing or mastering will be wasted. It's essential to remember that the music, more specifically the emotion of the music and what that does for the listener, is the most important thing. Especially if you're a musician who writes, records and mixes all of your own material. If your head gets swamped by a track, you lose your place or get fatigued and can't see the wood for the trees then take a step back and remember that as a musician or an engineer, it's your job to make great music.
So what do we mean by "everything else"? Well, everything else includes all the technical aspects like mixing levels, applying effects, equalisation, compression etc., all the post-production components. But everything else could also include things like distribution, promotion, social networking, other industry aspects or really anything that isn't the actual song. If you're getting bogged down or spending too long on these tasks, it could be a good time to bring the focus back to the music, to concentrate on questions like, "Are these tracks really strong?", "Is this the kind of music I would listen to?" and "Is the musical message clear and being communicated well in this recording?".
I'd like to underline it's not that these elements aren't important, they are. Many would argue that the perfect piece of music would excel in all aspects: a fantastic song, recorded, mixed and mastered well and promoted so that it reaches the intended audience in the best possible way. I would agree that these things are important and I'm not discounting them, but by saying "Everything else is secondary" the emphasis is on the proportion: How important the actual music is and consequently how much time and effort you should devote to the music versus all the other tasks. If you're a musician that also records and produces, then maybe an 80:20 ratio is a good place to start, 80% of your efforts on music and 20% on everything else. If you're solely an engineer then perhaps you can steer your thoughts to focus less on technical things and more on the question, "What's best for this piece of music?".
Everybody's different. There's probably no need to spend too much time on coming up with your own perfect ratio of musical efforts vs. other tasks (indeed a task like this would fall under the "everything else" category). The main thing to remember is that having a great song is much more important than a great sounding mixdown, master or indeed any other technical aspect. Keep your focus on making great music and you'll find that the other elements will fall into place much more easily, it's a pleasure to mix or master a great track and you'll also find you'll finish more songs more quickly.
Make great music. Everything else is secondary. Try to keep this at the forefront of your mind at all times. It's so easy to forget.
And go make great music now.