By the way, I don't know why the site says this match is from lichess. It's not, I just imported my match from here to that website to analyze with Stockfish.
What would you have done differently?

12. d5 is a bad move because it invites black to close up the position (aka fixing the pawn structure) with e6-e5. It also locks your DSB out of play, and gives the black knights a juicy outpost on c5. Similarly, 15. e4 isn’t great as it further fixes up your pawn structure. Black could have dominated the dark squares with 15...Nc5 and 16...Nh5.
12. c5! is a much needed pawn break that opens up the position. An example: 12. c5 d5 (Black wants to keep the center closed) 13. c4! (No way!) c6 14. cxd5 exd5 15. Bd3 White has a space advantage and is going to play e4 in the next few moves.
BTW, you failed to take advantage of black’s early queen moves in the opening. 6. h3 forces black to waste even more time rescuing his queen: 6. h3 Qg6 7. Bf4 (developing with tempo) d6 8. e3 Ne7 9. Bd3 f5 (9...Qxg2? loses to 10. Rh2) 10. O-O White has a huge lead in development.
Hey. I'm posting this thread because I'm trying to improve my positional play, but it looks like I'm just bad at it.
I know I won and all, but I don't really give a cent about that. I analyze all of my matches, the won ones too, and I'm aiming to master positional play.
So I'm white and everything goes fine until on move 12 I do a mistakes (analyzed with Stockfish), probably a positional one, and I give my opponent a -1.2 advantage which will last for most of the match. I know that's not a lot at my level, but I don't like giving my opponents such advantages, especially if they're positional ones.
Take in consideration that if my opponent wouldn't have blundered in the endgame, he's most likely win with that advantage.
So, what would you have done differently? (Don't ask stockfish - give me your opinions and explain them). Also, how can I improve these positional mistakes? I really can't see how to improve my position in such "pawn" matches, and so I do these mistakes.
Thank you sincerely!
Game: