Silman's Amateur Mind and Reassess Your Chess might be good places to start.
Unsure how to make progress
#1
"Aside from when I make blunders/mistakes, the reason I lose some games is because I reach a position where I'm just not sure what to do"
++ It is not an aside: the reason you lose always is blunders/mistakes. Maybe when you do not know what to do you make a random move and that turns out to be some blunder/mistake.
"This leads to me basically guessing almost or just moving something that "looks good" which is probably not the best." ++ Always check your intended move is no blunder before you play it.
"I have a decent idea for the opening- follow the principles such as development, fight for centre, castle King to safety etc." ++ That is very good.
"Middle game you want your pieces on good squares and to decide how/when/where to attack." ++ That is right. You can only attack when and where you have some advantage.
"Sometimes, often when you've reached the middle game and it's a level position and you aren't sure how to break through the opponent's solid defence and you don't see any immediate tactics, there are times it just isn't clear what to do."
++ In a level position there usually is no way to break through. Improve your worst piece. Try to trade your worst piece for his best piece. Improve your pawn structure.
Aside from when I make blunders/mistakes, the reason I lose some games is because I reach a position where I'm just not sure what to do(and I'm playing someone who does know what to do in the current position). This leads to me basically guessing almost or just moving something that "looks good" which is probably not the best.
I have a decent idea for the opening- follow the principles such as development, fight for centre, castle King to safety etc. Middle game you want your pieces on good squares and to decide how/when/where to attack.
Sometimes, often when you've reached the middle game and it's a level position and you aren't sure how to break through the opponent's solid defence and you don't see any immediate tactics, there are times it just isn't clear what to do.