endgame analization please?
In Knight endgames tactics tend to be more important than theory.
Well, let's work backwards...
43.Kxb3 entered a drawn knight endgame. Your pawn was not very advanced and his king was closer to the pawn, so you never had a chance.
29...Nxg2 entered a roughly equal N+2P vs N+2P endgame.
31.Ng5? - I agree it's not the best move, but you say "and White permits a draw" and I don't see how you were planning to win against best play.
37.Kg6, 38.Kf6 wasted a tempo futility chasing the knight. I was looking at 37.Ne4 as an improvement since it gets your own knight out of the way (and besides centralizes the knight) when I realized 37.Ne4! is actually very strong. Black's knight only has four escape sqaures... g3, f4, f6, and g7. This move covers three of them! Your king would now be able to decisively chase off the knight and then you could advance the h-pawn. And best of all, this line takes advantage of that odd Ng5 move you made early on.
37.Ne4! b4?! 38.Kg5 Ng7 39.h5 Ne6+?! 40.Kf6 and White wins. It's not forced win, but 37.Ne4 gives you practical chances.