Lost at the end of the middle game. Need analysis

It is really upsetting to lose after playing fantastic(i believe) till the end of middle game. And i messed up.

The knight, at move 31, was holding the white squared bishop for ransom and so playing 31. Nd3 was a mistake.

Move 43 was a mistake which could have led to a draw.

Your analysis, comments, critique, suggestions, or even compliments will be greatly appreciated.

I analyzed this with Shredder 12, and the score went up and down, up and down, as it does in many of my games!  Some of the more questionable moves that I saw were:

13. Na2?  Why bring it away from the center, out of the action?

16. c3? I still can't really find a reason for this one.

18. exf would have been really nice, because it creates threats.

Obviously, the ending was lost, but those endings are complicated and can easily be blundered away.  No worries there, mate.  :)

Here is how I analyzed the position after move 12.

1. His Queen side pawn structure was strong while mine was weak. So I fugured he would want to apply pressure on c2 by doubling his rooks and then exchanging his weak black squared bishop with my c3 knight.

2. My e4 pawn was just standing there in the centre and wondering what to do next. I had no clue, how to deal with it. Move it ahead or get f pawn to go with it.

I played Nc3 so that I could c3 and place my knight on b4, ending his threat on the open c file.

I did not play exf because after Bxf6 and with two passed center pawns, he would gain advantage over me.

at move 31 i am thinking Nc2 then Rd1 and start piling up on the isolated d5 pawn.  once you win that, you will have the queenside majority of pawns.

Ya, Thats a good idea.

31. Rd1 Rd6 32. Nc2 seems better as that would keep his knight pinned to the rook. If 31. Nc2 then Nc4 threatening the b2 pawn.

Thanks Skwerly and dark_niteb for your comments

Move 23, you should have taken the knight with your rook.

Thanks sicwidit. But 23. Rxe6 would make me piece down, as I had already sac'ed knight.

May be i should have played 20. Bf2 and let him taken my e pawn. And then get my knight onto the strong c5 square followed by b4.

you cant!

see?

EDIT: oops too slow...

after 30. move you just had to atack izolated pawn. Ind to nade a plan how to do that