Basically it's because you pushed your passed pawn to its doom. It should've been held back and pushed perhaps later in the ending. Other than that I thought you played pretty well considering the huge rating gap
Pyromania!!!
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Honestly, post title + rating made me expect much, much worse.
I think the biggest mistake is 30...Rdd8 ?. Although the game is probably lost at that point (White can double on the 7th no matter what Black plays) it is still a very bad idea. In rook endgames, activity is almost everything.
Of course there is a mate threat to counter, but ...Kf8 looks better (even if White plays Rb7 with the idea Ree7, and, again, Black is probably doomed).
For a while, my chessboard was ablaze here (at least for me), but then I got BURNED! How did my valiant Icelandic opponent get the lead here? I have chess pyromania: an addiction to playing with fire when it comes to taking on higher players!