40.Ne6 Ne8 or 40...g5 41.h4 h6
Where did I screw up?

He didnt lose this game because of the opening, he was a pawn up in the ending and lost through bad endgame play.

42....Kb4 I believe is a good winning attempt for black. The black king should approach the kingside with the idea of winning the weak white pawns there using king and knight. The a pawn will be used as a diversion to tie down the white king or knight . Black is certainly clearly better at move 42, if not completely winning. In any event he certainly should NOT lose.

Thanks a lot guys. Special thanks to NM Reb for bringing attention to the endgame (where i most wanted advice) and to Fiveofswords for his advice on the opening. It wasn't what I wanted but now i can see that I needed it. after 2.c3 I have no idea what to play. Most people play 2.Nf3 and then i go for the Dragon with 2. ... d6. I just played d6 in the above game cause, heck, what else? But now that you mention it, yeah it doesn't do much for the center.

42...Ne4 is a subpar move me thinks.the knight planted on d6 obstructs his king from getting to your kingside pawns...so while he wastes time by chasing your knight with his king you can advance your passed pawn.

Quite interesting ending. You were completely winning after 46.Ke3? Consider this position:
The first thing to notice is that white will have to try to sac the knight for your passed a-pawn before it queens.
The second thing is that your knight is in a good position to be sacrificed for the two white pawns; if you then manage to establish a pair of pawns on f5 and h5, white cannot prevent one of them from queening (a well-known pattern).
The only problem in the position above is the white knight that prevents you from playing Nf5. So it is most important to force that N to go to the queenside as soon as possible and place your king where it will be able to chase the N away. The key moves here are advancing the pawn to a4 and putting the king on b2, in either sequence. If white K goes after your N, you should be able to bring it back to the queenside, chase the white N away and promote the pawn. If white K returns to help his N, then your N will rule on the king side.
After 47.Kf4?, the white K is out of your pawn's square, so you should've gone after his knight. 47...Nf1 was a mistake. White could've immediately played 48.Nc6+, sac the knight, and have an easy draw.
In the next couple of moves, you had the good idea: once the white K was away chasing the pawns, queening the pawn should've been the first priority. Just remember to chase the N so that he cannot check you immediately, so, for example 50...Kc3! instead of Kb3. Also, bring in your N with tempo by playing Nd2 when possible, to take away some squares from the white N.
After 55...a3, the game was a draw, fairly simple to calculate. The next couple of moves are forced, since white has to sac his N or lose. The plan for black should be to sac the N for the white f-pawn and either get the king on g8-h8, or block the white K from leaving the h-file. For example:

@Wustenfuchs ... you screwed up the ending. You had a won ending. I think the correct plan was to centralize your King and N... attack his pawns and exchange Ns at the first opportunity... at which point your extra flank pawn gives you an easily won K & P ending.
@LegoPirateSenior: "Quite interesting ending. You were completely winning after 46.Ke3?"
No I don't think that's true at all. By move 46 Black's N has been terribly misplayed and will have no squares after Kf2. White misses this -- but Kf2 wins the Black N (so 46...Kb2, or ....a4 fail to Kf2 Nh1 Kg2 etc.) -- and the draw is secured. The White king isn't out of position for draw because the Black king can't promote the a-pawn by itself against the White N (see diagram below). In sum, 46... Nf1+ is forced at which point Black still has chances but it's tricky. Not "completely winning" unless you are completely accurate.
Yes, I don't like 42. Ne4 as well. After that your knight headed toward the edge of the board and away from the center when you had that wonderful advantage of
the outside passed pawn.
Also, 55. a3 allowed the knight sacrifice for your pawn which is a fairly common
endgame tactic. It was a nice ploy by white as soon he had two passed pawns!

@LegoPirateSenior: "Quite interesting ending. You were completely winning after 46.Ke3?"
No I don't think that's true at all. By move 46 Black's N has been terribly misplayed and will have no squares after Kf2. White misses this -- but Kf2 wins the Black N (so 46...Kb2, or ....a4 fail to Kf2 Nh1 Kg2 etc.) -- and the draw is secured. The White king isn't out of position for draw because the Black king can't promote the a-pawn by itself against the White N (see diagram below). In sum, 46... Nf1+ is forced at which point Black still has chances but it's tricky. Not "completely winning" unless you are completely accurate.
If white plays Kf2 and then Kxg3, winning the black knight (I think this was white's plan when 46.Ke3 was played), then the black pawn queens:
Now all this requires fairly deep calculations, which are easy if you can analyze the postion on a side board. In live chess, it is likely that I would not be able to do such analysis.
This said, I definitely agree with the earlier comments about move 42.
First, as rightly mentioned by JG27Pyth, a king generally cannot secure a pawn promotion against a lone knight.
Second, a lone knight cannot capture a passed pawn -- it can only stop it.
Therefore, the presence of black K near his pawn was superfluous. Both black K and N should've ganged up on the kingside against the white pawns and king.

@LegoPirateSenior: "Quite interesting ending. You were completely winning after 46.Ke3?"
No I don't think that's true at all. By move 46 Black's N has been terribly misplayed and will have no squares after Kf2. White misses this -- but Kf2 wins the Black N (so 46...Kb2, or ....a4 fail to Kf2 Nh1 Kg2 etc.) -- and the draw is secured. The White king isn't out of position for draw because the Black king can't promote the a-pawn by itself against the White N (see diagram below). In sum, 46... Nf1+ is forced at which point Black still has chances but it's tricky. Not "completely winning" unless you are completely accurate.
If white plays Kf2 and then Kxg3, winning the black knight (I think this was white's plan when 46.Ke3 was played), then the black pawn queens:
Now all this requires fairly deep calculations, which are easy if you can analyze the postion on a side board. In live chess, it is likely that I would not be able to do such analysis.
This said, I definitely agree with the earlier comments about move 42.
First, as rightly mentioned by JG27Pyth, a king generally cannot secure a pawn promotion against a lone knight.
Second, a lone knight cannot capture a passed pawn -- it can only stop it.
Therefore, the presence of black K near his pawn was superfluous. Both black K and N should've ganged up on the kingside against the white pawns and king.
You're right, 47.Kf2 is a mistake there... White has to stop the pawn with the N..
It gets extremely tricky (at least for me) to convert the win. We can all agree that in no way should Black ever lose, however!
Here's a game I played recently on an ICS. I got a slight advantage in the endgame and feel that I could have capitalized on it. (Or at least gotten a draw.) Instead I had little choice in the end but resigning? Where did I go wrong and what could I have done instead. I'm not concerned about the middle game. I feel I did good enough there. It's just after move 39 when I finally got the advantage that I'm concerned about.
My main suspect is 55. ... a3? but what do you think?