At 1200 no one is playing 20 move long theory so drilling moves will probably not help much but understanding the plans of your opening is important aswell. Anyways I hope you are doing well!
mistakes and blunders around moves 10-15
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I recommend you to learn how to spot tactics for the middlegame (for example with puzzles). Also, look always before you make a move if it doesn't blunder any piece and check if the opponent has any hanging piece.
Do you know what the middle game plans are for the openings you play? The purpose of the mobilisation to the squares is to enact a plan. So blunders stem from not understanding the plans from both sides. How they can attack or defend against an attack, whether you should be going into an endgame, or even that it is an endgame not a middle game and that you should be doing something else.
It probably results from playing too fast.
I've been playing chess for a year now and currently rated ~1250.
when i review games i lost, i always make mistakes when transitioning from the opening to the middlegame, around moves 10-15.
Do i need to learn more openings to avoid this? i currently only know the scandinavian and the Ruy Lopez opening.