Postdoc in good environment, but haven't accomplished anything on my own: How long is it reasonable to stick around?
Welcome to the club of being a miserable postdoc!

I was and am in quite a similar situation.  I had the benefit of my first postdoc being with a horrible boss and lab, and my second being with an amazing boss and lab, so it has been easy to figure out which aspects of postdocing are me sucking and which aspects are the environment sucking.  During the second postdoc with the good people, it became clear that the persistent lack of passion was on me.  Even after I finally saw a few projects to completion, the emptiness remained.  I think if you're feeling consistently unhappy, it's a good sign that the academic research life isn't for you.  There are tons of other career paths where you can engage in the activities you do find fulling (mentoring, teaching, counseling, etc.) that don't require slogging through miserable research hours.

It also sounds a bit like you have a touch of imposter syndrome.  Read this for some tips on how to work through it.

I'd hold on to the postdoc at least a few months longer while you start investigating new options and figuring out finances in case you need to go jobless or re-train for a bit.  Once you've figured out a plan, just be honest with your supervisor-- it's unlikely to be the first time they have had a postdoc jump ship.