No need to play 15...Nxf3 imo, your knight is strong there so I would have left it on d4.
He retook with 16. Qxf3 and then missed 17. Qxb7, maybe he just didn't want to open the b-file.
I think 20...f5 was a mistake, probably the mistake of the game. You weaken your control of e5 which really comes back to bite you, and it loosens the squares in front of your king, where his pieces are gathering. It also opens the file so his queen is pointing right at your king.
I agree with smartens that 20 ...f5 was probably the biggest problem. What was your thought process at the time? Just to kick the knight out? In general, it's not a good idea to move the pawns in front of your king in the face of an attack, as they provide the best (and usually last) line of defense.
Also, you chose to play for a Q-side advance, which is fine with opposite side castling, yet the c pawn never left the 2nd rank. There are a few other things as well, like doubling on the d file, that weren't so bad but added up.
Thanks for all your input. I improved my game from your advice.
So thanks again.
Can you guys tell me some alternatives to g6?
You're trying to defend g6 since he has 3 pieces pointing at it and you have 2 (rook and king). If you move your dark bishop anywhere, that adds the queen as a defender to g7, and if you move the dark bishop to f8, then you have 4 defenders for g7.
I got to a good position but it all went downhill from here. Any advice on how could i have turned the game around?