The engine here on chess.com isnt the strongest one...
I had one of my games analysed with the engine and it suggested variations which was inferior to the game continuation.
You shouldn't be so hard on yourself in the opening here, 3. g3 for example is a very playable move, white wont get much of an advantage but its a totally okay variation for white to play if he wants a safe and solid position.
6. b3 is also a fine move, the bishop doesnt really have any good squares along the c1-h6 diagonal and its perfectly resonable to fianchetto the bishop here. The f4 square is a very unsafe location for your bishop because black always has the threat of going Nh5 chasing the bishop and the bishop might also be a target when black playes hes freeing move e6-e5.
Your opponent however playes one mistake in the opening and thats the move 4...e6?! When black playes c6 and d6 he usually want to break with the move e5 and the move e6 is simply slowing that plan down. Now you can probably figure out that 6...Nbd7 was the right idea from black because hes preparing to play e5 (maybe c5 also) and the bishop doesnt want to go to the square d7 anyway so theres not much point of manuevring a knight to c7.
Btw, what is the Przepiorka Opening? I have actually never heard of it.
Here's a game I played recently. I had it analyzed by the chess.com engine, but I included my own annotations because I disagree with a lot of the computer's analysis.