Before you even get started mixing your track, you need to know what you are jumping into. The very first thing to do is listen objectively to what goes well and, more importantly, what doesn't.
Ideally you can approach this part with a fresh set of ears, though, if you've spent the last 20 hours composing and arranging the song, you may not have that. In the case of the latter, I'd recommend starting a new song or at least taking a bit of a break before getting to work.
You can of course fix a blatant issue if it is obvious immediately, but since mixing is likely to take a couple of hours if you have a busy track, it is best to have fresh hearing.
Notepad plugins are great for taking notes before mixing
and even after, especially if you are working with others.
Once you have this, you can grab a notebook, or even better a notepad VST, and note everything that is wrong. Consider adding solutions eg:
Gtr 1: not sitting well under rhythm >> compress and carve out low mids
Vocals: not clear at 1.03-1.45 >>> boost air and trigger synth sidechain with vox
Even if you don't cover everything, having a few notes will really make a difference. It will allow you to have a direction instead of slapping on another compressor just to see if that fixes the issue.
It will also allow you to gain a strong understanding of fixing the issue, after practice you may be able to fully mix the track from the plan, just by hearing everything wrong and identifying the solutions.