Basic Tactical oversights, missing endgame fundamentals and general garbage.

This looked like an even game to me up to about the pin 21. Bg5. Black was able to unpin that pinned knight quickly with a check (21...Ng4+), which not only rendered that bishop move useless, but forced your king to the kingside to protect your h2-pawn. You had probably run into time pressure at that point. As a result of that shift of your king to the kingside, your king was unable to adequately protect the queenside pawns, which Black exploited by seizing control of the b-file by doubling rooks there, combined with a bishop aimed at the isolated a-pawn there. After that a pawn had to fall, which likely would have been decisive.

Alhough you managed to get your king back to the queenside to protect the c-pawn, that shift took a few moves, and Black expertly took advantage of that time period to pile on pressure on the kingside (this strategy is called "alternation."), such as using one pawn (Black's h-pawn) to blockade two pawns (g-pawn, h-pawn), which gave him a winning position. In retrospect White probably would have lasted longer just giving up the a-pawn. This was a rather instructive game, I thought. I had to play through it a few times to determine cause-and-effect.

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(p. 25)
Alternation   Relying on a spatial edge to shift attacks between two
different enemy weaknesses until the defender must make a conces-
sion. A term used by Aron Nimzovich (1886-1935), a great player and
influential theorist.

Pandolfini, Bruce. 1995. Chess Thinking. New York: Simon & Schuster.

I'll just pick one move to comment on, and that would be 40. Kd1, I would have played Kc3.  Keeping the King trapped on the first rank like is rather advantageous in a endgame.

@sqod

You pegged the problem as 21. Bg5 not 32. Kd2 instead of h4 or 37. Re7 instead of Kc3 and going after the d pawn while leaving the rook on the second file?

The other posters noticed additional weak moves I hadn't. I agree with their comments, too. I just mentioned the point where it looked like White started going on the defensive in an obvious way. I'm not familiar with that variation of the Scandinavian--I don't play 3. Bb5+--so I don't know if there were even earlier opening mistakes.

I did specifcally ask for any comment, but the point of Bg5 was to double Black's pawns, and Ng4 does not prevent this. I handled the endgame poorly, yes in part because I did not have time, but still poorly; Bg5 was almost certianly not a mistake.