You held a level endgame for fifteen moves, then walked the king into a mating net.
This is the first game in a while that reached an endgame, and the eval curve says you defended it. From move 23 onward the position was level, and it was still exactly +0.0 when you played move 39.
21...Ng4 - the knight goes out and comes straight back
Thirty-three seconds, and -0.2 became +1.0. The mechanism is on the long diagonal: with the knight off f6, the bishop on c3 hits the pawn on g7, which is why 22...g6 was needed a move later, and then 23...Nf6 put the knight back where it started. Two moves spent, a pawn move you did not want, nothing gained. Rxd1 was the move: Rxd1 Rxd1 Nd5 Ba1 Ne3 Rc1 g6 Nd4 a6 Re1 holds -0.1, trading a pair of rooks first and then routing the knight to d5 and e3, squares it can stay on. This is the wandering-knight pattern again, same as Nc5-Nd7-Nb6 on 07-22.
39...Kxg4 - the king takes a pawn on a square two pieces cover
Six seconds, +0.0 to +2.6, and this is the move that lost the game. The board facts are checkable in four seconds each: with the king on g4, White has Bd1+ from b3 and Rg1+ from a1. Both were available immediately, and 40.Rg1+ was played. Bf4 was the move, leaving the pawn alone: Bf4 Kf3 Bd2 Rh1 a2 Bxa2 Ra3+ Ke4 Bxb4 Bd5 stays at +0.0, with your a-pawn running and the rook checking from a3. Then 40...Kh5 went further in for two seconds, +2.3 to +5.0, onto a square where Bd1+, Bf7+ and Rh1+ are all on; Kh4 was about half a pawn better. Taking the pawn nearest the hand without asking what the square gives away is the habit costing you the most games, and it was 16...Qxe5 on 07-23 in queen form.
42...Ra7 - the last defender leaves and Rh1 is mate
Seven seconds at +5.9, and it allowed 43.Rh1#. The rook on a8 was the only thing defending the bishop on d8, and moving it also cleared nothing useful. Bxf6 was the try: Bxf6 Kxf6 Kh4 Ke5 Rb8 Ra1 Rxb4 Rxa3 Kg5 Kd6 gives back the bishop for the f-pawn at +5.5, losing but with a king that can breathe.
What to drill next
- The position before 39...Kxg4. It is dead level and there is a pawn on offer. Hold the engine to level from here three times running.
- Exposed king puzzles, for the habit of counting the checks a square hands over before stepping onto it.
Before a capture, name the checks the new square gives them.