I have been breaking even for close to 80k hands now at 100NL/200NL. I know it's due to poor play, and I kept thinking it's due to my inexperience, that I needed to study more, which is definitely true -- but I feel pretty confident that it's far from my biggest leak:

Playing while not in the right mental space.

For me this includes

-Playing on auto-pilot
-Playing while tired/drunk/stoned
-Playing on life or poker induced tilt
-Playing while trying to watch TV
-Playing while my mind isn't 100% into grinding

All of the above result in not having the mental focus to properly think through hands. It's like I had this attitude that the low stakes are so easy that I should be able to turn off my mind, 12 table, nuthunt and profit. When my mind is turned off from poker, I don't think things through postflop and end up making mistakes for my stack -- not to mention making loads of smaller mistakes.

I've read dozens of times, don't play when you're upset/tired/intoxicated etc...and for some reason I never took it to heart. Probably arrogance. What finally convinced me was watching my non-showdown profit graphs. When I play in a 'thinking' mode, this graph is near flat, slightly positive. When I play on auto-pilot, the graph is steeply negative. Over a couple of months I've noticed such a direct correlation with this graph and my mental state that I can't ignore the evidence anymore.

So now I'm taking a quality over quantity approach. I'm only grinding cash when I feel in a near-optimal mental state, which means I'm probably only going to get in 20k hands a month instead of 40k hands, but 20k hands of winning poker is infinitely better than 40k hands of breakeven poker. ...rakeback ain't that good y'know?

So wish me luck on my first solid winning month in a while -- it's started great so far (10ptbb over 5k hands with a winning non-showdown graph). And To anyone struggling with the game: think hard about how mentally focused you are during the average grind session. This just may be your biggest leak as well.