Sounds like your either too aggressive, getting committed to hands that you picture as unbeatable, or not aggressive enough, not extracting value from premium cards. If you've studied the books, listened to the advice and put it into practice i'm guessing your hand selection is ok, and playing often is always a great way to learn (especially your from mistakes).
If not aggression, then perhaps limits. I would never advise anyone to play out of their limits, dont EVER risk more than you can afford, but the lower levers (I'd even say lower than $20 buy in) can be a real challenge. 75% of players will play any 2 suited, any 2 connectors, any Ax or Kx, and it leads to bad beats and suckouts i'm afraid. Good luck anyway, i'm still learning myself and not profitable after 2/3 years play!