Some of the mistakes are so unbelievable that they really are just that. For example losing a queen to an opponents position which I should have seen beforehand.
My frame of focus is not on the full board always. Mostly it's on the action. And so, I sometimes lose to a non developed piece also. Simply because I never saw it! Because it had never moved the entire game!
If you lose the pieces by simply not noticing that they can be immediately captured, I would suggest systematically checking:
If you don't have time for this, I'd suggest playing longer time controls until it becomes second nature. It is completely normal to miss these kinds of moves in the beginning. Try to establish a habit of systematically checking for threats, until your board vision is developed enough that you notice them subconsciously.
Based on your description, you might consider using Step One[1] of the The Chess Steps[2] to improve your board vision to improve your ability to
In your case, the regular workbook, Step One Plus, and Step One Mix would probably be appropriate (more info on The Chess Steps). In the Mix workbook, you don't know the theme so you may have a checkmate, one side may have an undefended piece, or you may need to defend, etc.
[1] Step One: https://www.chess-steps.com/books-step-1.php
[2] Chess Steps: https://www.chess-steps.com/home.php
The two best things you can do to improve your ability to realize hanging pieces, simple mate threats and so on are:
Tactics puzzles: these will mostly train your ability to recognize certain patterns (pins, discovered attacks and so on) as they appear on the board. Once you get to a higher level, it'll also be important to calculate ahead accurately so you can find the precise way to exploit those patterns.
Game review: after you finish a game, go again through it and try to find stronger moves that were missed by either you or your opponent. At this stage I'm mostly referring to oversights (like an undefended piece nobody noticed, an easy available fork...)
The answer may not necessarily be as exciting as you'd hope but the truth is drill puzzles every day, at least 10, but as many as you have time for. What time control do you normally play? online its understandable most people don't want to sink an hour into a game, so play 10 or 15 minute games, getting over the board classical experience is great though, it will develop your thought process and pattern recognition more (in conjunction with puzzles and general theoretical study, like openings and endgames). Additionally, in classical you have much more time to do a more in depth 'blunder check'.
