I'd have taken the bishop too. Wonder what black saw differently? Is it that after 12. ... BxB 13. QxB is with tempo on the bishop on c3?

12...Bxd3 would have been very difficult for white to draw but 12...Bxf3 giving someone like me light square dominance would convert this bad position to a draw.

I agree: 12...Bxd3 was indicated here. But when a strong player is faced by an opponent much less experienced, 'second best' moves can turn out better than 'best moves.' Sometimes...

If you can't find a good move, look for your worst piece and improve it. Your bishop on c1 is sitting at home doing nothing. It needs to get into the game somehow.Not entirely easy to figure out how, granted. 14. e4 leaves a big home on d4 for the knight to occupy. 14. Ba3 achieves little, and 14. Bb2 swaps another set of pieces off. But maybe it's still the best.

Wow, if you knew that keeping a good pawn structure in front of your king then you would know not to open lines in front of your king! Best was 14. Qd1 e6 15. Be2 and black has a tough nut to crack.

Yep: much too weakening. I might have been inclined to begin a regrouping by 14.g3!? with the idea Rd1, Bf1 and Bg2. Alternatively offer the exchange of dark-square bishops by 14.Bb2. Again, not especially desirable, but it does confront Black with the decision to exchange bishops with a consequent weakening of the dark squares on his K-side; or ceding the dark long diagonal outright. A possibility to consider is the prospects in a 'pure' major piece end game after 14.Bb2 Bxb2 15.Rxb2 Ne5 16.Qe2 Nxd3.

You all make great points. It shows I still have a lot to learn. I agree, this was not a good move on my part.

And here you can see why this central domination is important, black has a huge space advantage.

Here e5 is only possible because of 14. g4. For example, if 14. Qd1 e5?! then 15. Qc2 saves the day!.

I think the earlier d3, losing a pawn and giving up this big centre, has to count as a decent-sized mistake anyway. But then, cutting out mistakes is one way of becoming a stronger player - the ratings gap here almost predicts that you'll make a big mistake somewhere along the line which will cost the game.

In some cases being a pawn down is not such a bad thing as long as you have compensation for it. Such as piece activity, central dominance or space advantage just to name a few. In this position if white has some control of the centre black's pawn advantage many have not been so great.

Black is material ahead - so swapping off pieces makes sense.