I left in the middle of my year 11 but decided to work instead... but is finnishing your year 12 certificate the most important thing while your young?

Nothing wrong with work!
A person has to eat.
Finishing your certificate would have been good, because you might want to pursue
Further education later on.
This is usually a pre-requisite.
It depends very much as to what your future intentions are.
Good luck.

Rolcam.