Positional ideas in this middlegame

I just played this game: https://www.chess.com/game/live/131319553483 and I would like to understand what kind of positional ideas I should have played for in the middlegame. Is anyone willing to review this game for me?

To put the question a little more specifically, it was move 19. f5 where I was kind of lost on what to do. I figured the knight on e4 was strong so I tried to kick it, but the engine disapproves. Yes, I know there is a button called 'Show Moves', but it's not immediately clear to me why the given line is good.

EDIT: I was playing with the black pieces.