I don't see anything terribly complex or subtle. Rather, I think you've just overestimated your position -- IMO, Black has equalized by the 12th move. Doubling black's pawns in the Sveshnikov with gxf6 is typically more of an asset to him than a liability. With the queens already gone at that point and your Bd3 not doing anything, there's no real danger to him in your attack. Eventually you trade down to an endgame where his bishop is superior to your knight, and then after that to one where his rook & king are more active than yours, QED. Not an awfully played game or anything, he just nicely converted his advantages into a win.
Where did i go wrong?

You played great. Probably you had the better of the opening but then by the middle of the game it was pretty even. Probably it should have been a draw but you lost a technical rook + 2P endgame.

when you traded down, you left him/her with a more active rook and slightly more easily protected pawns. Also I think instead of 15. Rg1 you should have played Bc4 not allowing him/her to trade knight for bishop and giving you a backwards d pawn.

I think 9. Bc4 Qb4 10. Qxb4 Nxb4 11. Bb3 would have been better than 9. Bd3.
Bd3 was just unnecessarily passive, restricting the movement and scope of your bishop. It also forces the ensuing queen exchange (a normal manouevre in this opening is for the white queen to drop back to d1). I was going to suggest 9. Bb5, before I noticed 9...Nxe4!, which throws a spanner in the works.
The 11. Bg5 and 12. Bxf6 combination is interesting, but I'd say it would be better to hold onto the bishop pair. You can't really take advantage of the open file and doubled pawns, especially since your d3 bishop is hemmed in and the knight is a long way from the action on the kingside.
Likewise, instead of 14. h4, you could try 14. Be2, to avoid going into an open endgame with a misplaced knight against a bishop. After Be2, black's knight on b4 would be misplaced, and it would also prevent the d6 pawn push that releases black's bishop on c8.
Instead of 17. g4, maybe 17. Nd5 would have given black more to think about.
After about move 18, I'd say black has a slight advantage due to having the bishop and your knight being misplaced, so no surprise he went on to win.
i don't think 14. h4 is good, it doesn't do anything and cost you a tempo.
Bc4 was better, but Bd3 is playable too. I Don't like 21. Ne2, I think Nd1 and then Ne3 is a better idea. From e3 it can support attack later on d5 and f5 and it also
controls the c2 and g2 against the possible penetrating rooks. i don't understand 24. Rgh5, i think better is: 24. Rxg8 rxg8 25. rc1, rg2? 26. Ke3 and white is first to attack the a pawn.
I think the bishop on e6 defending the f7 is just too strong to do anything usefull on the king side, once the g and h powns are gone. The better breakthrough was over the c-line I think, but it is easily to say afterwards.
My conclusion is that you pushed too soon on the kingside. First placing the Knight to e3 was a better idea(On the right moment you could try Nd5/Nf5).
A end game idea would maybe something like this:
- place the rooks on the c-line
- keep the kingside closed(pawns on dark squares g3 and h4 ?)
- try to penetrate with the rooks through the c-line
- if this doesn't work try to trade them because the knight should be stronger in the closed game
- an idea might be to penetrate with the king through the following squares: b4-a5-b6-c6 and so on.
Just my two cents =)
What about 42. a5? I don't see the point of this move. I would have gone for 42. Rd6+, trying to reach a Philidor-type position (which I think it is possible). I think that 42. a5 let Black improve his position, while not making any threat (White's a pawn has a long way until becoming dangerous).

I dont think you can win an equal ending with claypot at the present if this game is anything to go by. 32 Rh6 was poss weak though it may take much calculation to prove, the theme of protect your second rank can be easier to follow than long lines of analysis. Whites 19th position may have been reached a move earlier if Kd2 instead of castle queen.
I have played this opponent 5 times now, mostly i give him a fight, but i always lose in the end. This game gave me 0/5 against him, i felt like i had a strong position but yet again i blew it. Please help me to understand where i went wrong, i know he is much higher rated, but i feel i have it in me to beat him one day, and i will keep trying :-)