Good OP NWF.
Dealing with any hands that are non premium is where the majority of poker is played. It is the premium hands, and the premium hands vs premium hands that get the majority of the 'press'. Tabloid poker.
But when and where to play less than premium hands is where most of us play, most of the time.
IMO the ability to play the NPH's (non premium hands) is always conditional;
-You need to believe you can avoid chasing draws at unacceptable odds
-You need to believe you can lay down decent holdings.
-You need to have shown the ability to bluff. Not so much that you have shown a bluff to the other players at your table, but more to ourselves. By and large, much of the time I have no 'bluff blood' in my veins. Oddly I almost always have at least some kleptomaniac blood (stealing) coursing through those veins.
-You MUST understand position.
-You should have a good read on the players at your table, or at bare minimum, believe you have a good read on them.
-You should be aware of your current table image. ***** Much harder than you might think. I consider myself primarily tAG and I believe there are more than a few members here who would gag on my own self-description.
-You should have a fair to better notion of EV. And in tourney play some awareness of ICM.
Of course the better at each of these you become, the better you are likely to succeed. I added several ideas here with the notion of believing in yourself, which will come mainly from the experience and self-education (books, articles, forum posts) that you expose yourself to.