How to improve my middle game
think about what your opponent will do and what you could do in your next turn
As for your middle game plan, assess the position whenever possible (it is hard when you have little time, but it is effective). For example, when the position is solid and both players have same material, analyse the pawn structures or strengths and weaknesses of pieces. Make sure you have developed pieces properly and put them on good squares, while try to force opponent pieces to bad squares. Learn not to kick bad pieces and leave them on bad squares (for example when a knight is on the edge of the board, there is no need to kick it off and give it time to go to a better square).
If you find opponent positional weaknesses, move pieces there and try to make pressure (instead of one-move-threats). Eventually they will crumble. Assess the pawn structures and break pawn chains of opponents whenever possible. Remove isolated pawns of your own by offering pawn trades.
You main idea in the middle game is
1) win material for easier endgame
2) prepare pawn structures for endgame
3) make attacks