I have no idea why I lost this game...

chessicstudent: I suggest reading up a bit on the Nimzo-Indian. It's a strong defense to d4.

Your 4. a3 is the Samisch variation. While it's not currently popular, it's sound enough and more than sound enough for chess.com play. The usual Samisch approach is based on 6.f3 not 6.Bf4. From your archive I see you often play 6. Bg5 and that's a better move.

This is my first official chess game in forty years and my first over the internet. I'm not that happy with my play either. I played too cautiously.

I haven't done a Fritz analysis of our game. I think I may have muffed 11...c5. I worried I might lose the exchange on my queen rook, if I wasn't careful, because of your bishop on f4 and the possibility of Bf3. I addressed that concern with 12...d5, as well as opening further attacks on c4.

12.Re1 seemed to be a placeholder move and better was probably available.

One of my goals here is trade off our light-squared bishops, which you allowed with 13.cxd5.

17. dxc5 wasn't necessary and I didn't mind seeing it.

I don't think 23.c4 was that bad. That pawn was in trouble no matter what.

chessmickey is correct that not taking 23...Rxf3 was a major mistake on my part. The combination looked good but I lacked confidence and was worried about time. I stuck with my strategy to win the c pawn and grind out the ending. That might have been hard even a pawn up.

No, you didn't make any major blunders at your level. The next level you face is positional, especially since you play queen pawn openings.

Tons of awesome analysis on this thread; thanks a lot, everyone! And, of course, kudos to my opponet, ipcress12 for a well-deserved win.