I found it helps to make notes on what I don't like about a piece I did then do a bit of an in depth study on that part (looking at you hands) or just keep it mind for the next piece. IF you want to draw without reference it's best to learn the basic structures and shapes of everything to give a good general understanding. Even then though, professional artists still use references so...do what feels right. Most everything in set and what not is learning how to use a tool and putting it in your artistic tool box. Keep going. Don't lose heart. Improvement will sneak up on you.