White has been left with a hanging pawn-couple (Nimzovitch's term), which can be very hard to protect.

Already restraining and attacking the junior partner of the pawn-couple.

capturing with the bishop when c4 gets played gets met with Bxc4 Bxc4 jkarp since white's light squared bishop is also aiming at that square. I like Qc7, if you allow c4 then white gets a tad too much space for my liking.

Correct bipolarcorner!

I'm less inclined to be critical of this move. True, it drops a pawn, but maybe White can use the time to drum up some kind of counter-play. ...

Finessing. Black stifles any real chance of counter-play before snapping off the loose c-pawn. White can do nothing about that. This move also discourages any faint hopes White might have had of effecting anything at f7.

Not a bad retreat, considering...

Instead, 15.Bb2 - which 14.Nd3 made possible - would have chased the Queen off, protected the d-pawn and freed the a1-rook to move to c1 at an opportune moment. White would still have a losing game, for sure, but could possibly have generated some play.

One will find this with a lot of games between players of widely disparate playing strengths. Having obtained a winning game, the stronger player will deliberately select objectively inferior moves. Sometimes this is to test one's opponent's alertness. Failing the test will shorten the game considerably. But another motivation is to crystallise out the essentials of the position, to remove the slightest complication that the opponent might find a way to exploit, and, as it were, to persuade the opponent of the futility of further resistance.

Yes Qe6 was possible, covering c8, but I was already planning to play Qe4 if the chance arose.

Though weakening, 19.h3 might have been worth a look, threatening 20.Bg4. But in fact 19.h3 would have been a poor move, as after 20...f6, followed by 21...Qe4 Black has too many threats. It is now very difficult to find any sort of reasonable move for White.

I think I might have chosen the more forcing 19...f6...

A sign of inexperience. This sort of thing is so tempting until you learn better. White lashes out to chase away the Queen, but instead drives it to where it wants to go, whilst at the same time - and this is the crucial bit - opens up a fatal self-inflicted wound on the K-side,

The g-pawn's thrust has left the White king entirely without protection.