Here is a game I played recently. What do you think were the key blunders/mistakes that I made?

If you were Black you should have played 21...b5 instead of 21...Ree7.You also missed the back rank mate.

Your problem was your play was too passive and defensive.  Go on the offensive.  You said you wanted to get rid of d5.  Should have done exactly that at move 20.  That creates a double pawn file for white.

I would not have made move 17 because of the consequences that followed. Better to drive the knight away with c6 and put your on knight on d4 or f4.

And instead of 14...Na5, 14...Nd4 is better.

Please be relevant, helpful & nice!

I think I was, wasn't I?  Besides, he asked, and I contributed.  Would a smilie help? 

You pretty much spotted your own mistakes, you could've saved your knight by b5 a few moves prior.

Good points by the other posters! You chose ( ..e5)  as the 1st move as black, so opening principles tells us that if you can play (..d5) in that opening without consequences you have equalized or over-come white's advantage of the first move.

This is a really annoying feature of the redesign: I spent some time on a post, but when I actually post it, all I get is the Please be relevant, helpful & nice! message! My text had vanished. I can assure you that my reaction was extemely relevant, but not at all nice! I'm sure it's something that I'm doing wrong, but I'm at a lost to see what.

I assume you just timed out without saving it by hitting Preview, Paul.

And OP, !! for 23 Qa5? lol

I think what went wrong in this game for Black was almost entirely in his attitude. White's pointless, time-wasting maneuver with his dark-squared bishop left Black ahead in development. His best move would probably have been 5...d5! capitalizing on his lead in development (even White's one developed piece is on a bad square!). But in any case, Black got a good position and kept it.

Black could have done better with 14....Nd4 followed by c6. This way he gets a super outpost for his knight on d4, while denying White an outpost on d5. But even after the move he chose, he was doing fine. 20...cxd5 was quite strong, and 21..exd4 looks practically winning.

But none of this reflects what was really going on in the game. Black was practically in despair from the first move! Every tiny threat from White filled him with fear, while he never seemed to recognize and make use of the good things in his position.  Here's what Black had to say: After 10.Ng5, "I felt a bit intimidated by this move." After 12.Nb5, "I got really scared by this move." After 17.Nd5, "It seemed that my opponent was really going for it and attacking ferociously." Hard to believe that the player making those remarks had the better of it, but easy to believe that he lost the game. Black played good moves with a fearful, cringing attitude. He would have been better off playing worse, but playing with a purpose and looking for ways to win.

Thank's for the hint, Andy--I'll try to check that. Damn! This senility is a drag!

Yes, I definitely feel your pain there.  I keep hoping though that someday I'll get old enough to where I don't even notice that it's a drag anymore. 

Just try to learn from your mistakes.  The 3 main lessons I believe you can learn from this game are:

1)  Ask yourself before you move, what are my opponents threats?  (Missing Knight getting trapped & back rank mate simple mean you are not looking at your opponents threats before each move).

2)  As black, if white is not taking advantage of his initiative, you can usually make them pay by fighting back in the center.  I, personally, like 3 ... d5.  

3)  Try to keep the tension.  Taking with no good reason just helps your opponent develop & gain time.  Instead of 4 ... BxB, I'd recommend just d6, then Be7, then 0-0.

Learn chess ideas and try to apply them.  When you mess up, try and figure out if your ideas were bad, or just the execution.

Also, if you think your opponent is making a threat, ask yourself how there is a threat before you react.

Well that's certainly good advice for any player, but I don't think it really fits this situation very well. In the game under review, Black's problem was that he regarded every one of his opponent's moves as a threat. The slightest sign of aggression from White, no matter how harmless. seemed to cause panic.

So, while you need to be on the lookout for your opponent's tactical threats, it's also necessary not to be afraid of ghosts! A threat by your opponent isn't necessarily a real threat, and you need to learn to ignore the pseudo-threats and to push your own agenda.

On the other hand, it's not so much his demeanor as it is the moves he made and his general approach in the game.  You can't win by demeanor alone.  Knowing why he makes the moves and understanding board situations will be enough to change his demeanor.  But you can't be overconfident either.

Of course you can't win by demeanor alone: I glare fiercely at my opponents all the time, but it doesn't seem to help. On the other hand, attitude was what determined the outcome of this game. Black ignored all the favorable opportunities that were available and drifted into trouble and then finally overlooked a mate. But aside from learning to be a bit more careful about your back rank, there isn't a lot to learn from this game from a tactical point of view. But here's the thing: If you never try to do anything positive, and you react with fear and panic to every pseudo-aggressive move your opponent makes, you're not going to win. If you're scared all the time, you won't be looking hard for your own tactical opportunities, and if you just wait for the axe to fall, it will.Between players of roughly equal rating, attitude is very often the decisive difference, and I suspect this is true all the way up to the 2700+ level.

So true, I have talked myself out of a good move worrying too much of the the opponent threat. Ghost threats...

BTW: I may have missed it, but no one seems to have mentioned that although 23.Qa5 wins a piece, it's really a big mistake. 23.Rb8+ just forces mate in three. You really need to be more aware of your back rank. Once you move your rooks of the back rank, "accidents" can happen at any time. It's a good idea to through in h6 or g6 to give your King some air. 21...Ree7, the move you made because you couldn't think what to do, was losing move. It's funny how often that happens: you consider a few moves, you can't decide between them, then you suddenly make a totally new move that you hadn't considered at all, and it sinks you!