Well you weren't dropping material left and right, not at first anyway, which is good! That said, you eventually got totally lost about what to do, and it seemed like eventually your mental energy started to flag because of it. Opening your openings rook file when you are castled and he isn't is bad, so 17. nxg5 was bad. 18. h4 feels like an "i stopped paying attention because I have no idea what to do" blunder, it gave away a pawn for othing and totally cleared everything for black to attack you on the open file.
This game is not the best for teaching beginner middlegame principles. In general, if you aren't attacking the enemy king or defending yours, or gaining space with pawns, you should strive to improve your pieces, to make them more active or control more central squares. For the time being I think you should strive to complete development without fianchetoing your bishops. That is kind of a more intermediate concept and I feel like contributed to your not knowing what to do. So instead of 4. d3, play 4 bb5 or d4 and you can develop in a more straighforward fashion for a beginner. Those are also objectively better moves, since fianchettoing your bishop doesn't make a ton of sense here with your pawn on e4 already blocking the diagonal.
lol I'm just bad and I've been playing chess for a year now please help