Games where you are up a rook and a knight and you have a tremendous mating attack against a wide open king are not usually called "endgames." You obviously played the first part of the game quite well to get such great position, but then you fell apart. Were you very short of time?
One thing that would have helped was to play 26 Qh8 mate! Checkmate is the best way to solve endgame problems. You also missed a nice little combination: 29.Rxe4 Qxe4 30. Ng5+ forking king and queen. So I don't think your problem is related to the endgame. Missing middlegame tactics are your problem. They're almost everybody's problem!
This is why I need endgame practice! If this doesn't show you how bad at endgames I am, I dunno what will.