you've done nothing wrong, but white really played that brilliant sacrifice, i took a long time on this position, i considered h5, but busted by Rd7+. and i took another 2 minutes, i got Rd6, that might be the best move, i don't know if there is any threat for black to have after that move, but nice game still, time pressure is big in this game.
Where did I go wrong?
You put your Q & Rs out of play on the Q-side. King safety should have been foremost in your planning.
A few things:
First of all i think you did the opening very well and you have a good advantage(controlling the center an White playing passive without any counterplay). After 15. Ne2 Black is in very well shape, but now you make a few small mistakes. You played 15 ... Nd7 which isn't a good plan, only exchange pieces if you are cramped or get some direct advantage. Is like most 99% of the case you should never release the tention if you have the upper hand because it will make the task of the defender more easy. A better plan for example was:
15 ... Qd5 16. a4 or c3? (any move White has not many good moves to improve) 17. Rad8 with the ide of support of the centre while putting more pressure on g2(long diagonal).
17 ... f6? (You should now allow the White Knight on f5). After the trade you notice you start to loose control of important squares. Here g6 was needed.
After 18. Qg4 i have the feeling you panic a bit. The move Qg4 looks dangerous, but in fact it does nothing. From now on you play too passive in the centre and also make the mistake putting your Queen on the other side for some pawn grabbing. When you have the advantage somewhere make sure you first exploid this advantage before jumping into another plan.
Still the rest of the game your advantage is still clear, but the move which changes everything is 38. Rc7c6 is a blunder it changes everything.
Just my advice: Stay cool when attacked(most of the time 2 pieces isn't enough to attack your king). Your king was safer then you thought. Don't swap pieces or release the tention for no reason. When you have somewhere an advantage(center, lead in development) focus there first before you go out for pawn grabbing.

Mali-I think you are absolutely right. I was conscious of the fact that my power was on one side and my king was on the other, but I never found a way to activate those pieces on the kingside. Also, I have been abused by strong pawn pushers, so I was eyeing the 3 on 1 queenside pawn advantage of my opponent. So one of my goals was guaranteeing that this pawn advantage was neutralized.
Twisted-Exploiting my advantage should have been my main objective. Hell even in my notation early on I mention that I am very confident with my central advantage. However, it's clear looking back that I made no attempt to capitalize on this advantage in any way.
I rarely post games because I'm not big on bragging about a big win or posting losses because I usually have a huge blunder. However, this game was different. I don't have a game-changing awful blunder. In fact I feel like I outplayed my opponent, but that didn't mean anything because he converted the win because of the strong rook sac.
I've added some of my thoughts throughout the match. So my question is, where did I go wrong? Please add any other general comments.