Your mentor doesn’t have to be another writer but it could be. You want to find someone who will listen to your ideas and will be honest enough with you to tell you they stink if that’s the case. You need someone who will cheer you on at the same time they’re telling you the character doesn’t work.
It can also help to find another writer who inspires you to write your very best. It doesn’t mean you have to emulate them, however following their career and heeding their advice can help.