Beaten despite having what looked like an advantage at the end.

Hi! I've not been playing long (played a bit when I was a kid, and enjoying playing again now, despite the odd moment of head-banging stupidity). Had a game against a higher-ranked opponent, and I thought that towards the end I had what should have been a game-winning advantage. Would anyone like to tell me where I went wrong (either in the end game or earlier - I know there are a few foul-ups in there).

I've added notes through the game, as best I can remember. I figure that at turn 40 I should have a decisive advantage, but fluffed it. And 49.b5 was a mistake I'm still kicking myself over.

46 Nxh3+ seems better...?  But I am not certain of it.

Thanks, Bonesy1116. I didn't really think about pawn structure from 20.g5, but now you mention it it seems obvious that that would make a mess of things.

Thanks jonnin. I wasn't sure about the rook swap, especially with a pawn race going on, but I figured that the knight advantage would count for more with the rooks off the board. I was also a bit worried that I couldn't defend my pawns from a rook as easily as he could, as mine were in a terrible arrangement and distant from the King.

It is difficult to defend against multiple pawns by just having a knight. You must play precisely to win. It's a better decision to keep the rooks on the board as they are better defenders and can counterattack very easily.

I think 54... f6 looks like a mistake. 54... Nxb4 55. d6 a5 might work for a tricky draw.