Books can't tell you what you are doing wrong because books don't know what you are doing. Books can only tell you how to do it right.
chess is hard

Maybe you aren't reading the correct books or studying them correctly? Some books are very specific like Baburin's isolated d-pawn book or a specific CCE or ECE volume on a particular endgame type. If you need extra credit to really reinforce the point such as a certain center type just do a search for it in Chessbase and see how the winner implements their plan.
If you don't know how you lose and it's not due to lazy and/or careless calculation (leading to a loss of material at worst or giving your opponent a positional edge at best) then you have planning issues. What positions do you excel in? Which are duds for you? Tiger Lilov is a good coach and will help you form a study plan based off your strengths and weaknesses. Be sure to include quality hard fought losses as well as some wins.
because the books don't really tell you what you are doing wrong
the books are too general in nature