Your own pieces went loose twice, and the second time cost a won game.
Sixteen moves against a 1943 and the position was level, so the Caro-Kann did its job here. Everything that followed came from three moves.
16...cxb4 -- the a-file opens toward your own king
16.b4 hit the queen on a5 and the pawn on c5 at the same time, so something had to move. You spent twenty-five seconds and resolved it with the capture, and the position went from +0.1 to +4.2.
Qc7 was the move, and it is worth seeing what it also concedes. The queen on a5 was defending a7 down the file, and Qc7 stops defending it too, so the a7 pawn was going either way. The difference is the file it goes on. After Qc7 the engine plays Nf3 g5 hxg5 hxg5 bxc5 Nh5 Nxg5 Ne5 Qc3 at +0.4: White wins the pawn on c5, you get the h-file and the g5 and h5 squares for the knights, and the a-file stays shut.
After 16...cxb4 17.axb4 that file is open, White already has a rook on a1, and your king is on c8 at the end of it. The engine's own continuation from there does not even recapture on b4: axb4 Qc7 Rxa7 Nb8 Ra8 Rd7 b5 Rhd8 Nf3 g5, and it is still +4.2. The game went the same way with 20.Rxa7 and 21.Ra8+.
When a pawn trade opens a file, ask which king is standing at the end of it and whose rook is closer.
20...Rxd4 -- the rook on h8 loses its only defender
Taking back on d4 was right. The wrong piece did it, and forty-one seconds went by without the question that would have caught it. +4.2 to +7.1.
The rook on d8 was the only thing defending h8 along the eighth rank: your king on c8 does not reach it, the pawn on g7 defends h6 rather than h8, and the queen on c3 is blocked on the long diagonal by that same g7 pawn. Qxd4 does the identical job on d4 and gives up nothing. Both moves hand over Ra8+, so the check was not the difference; the loose rook was. The engine's line runs Qxd4 Nf3 Qd6 Ne5 Rhf8 Qe1 b6 Qb4 Ne8 Nxf7 at +4.3, which is still bad and still a game. Instead 21.Ra8+ Kd7 22.Rxh8 simply collected it.
This is the move you have been making. Against gayashan_desilva it was 13...Qb6, which stopped defending the bishop on d6 when Qc7 did both jobs at once. Same shape: two pieces could do the work, and you picked the one that was already holding something.
Before you choose which piece captures, ask what each one is currently holding.
26...Qe1+ -- three chances taken in six moves
From +7.1 you were lost, and the position came back because Drotciv let it go three times and you answered each one correctly.
21...Kd7 was the only move that held, and it is not a natural thing to play: walking the king up the board while a rook takes on h8, then Rxh8 Qxd2 Qxd2 Rxd2 Rb5 Rd1+ Nf1 Kd6 Rf8 Ke7. 25...Kc7 was the engine's first choice. Then 26...Qe1+, again the only move, going Nf1 Qxf2+ Kh2 Qxf1 Rf8 Nxc5 Rxf7+ Kb6 Qb4+ Kc6, with the queen eating f2 and f1 with check on the way.
That reading is reliably there when the loose piece belongs to your opponent. The next section is the same reading pointed at your own side.
27...Ng4+ -- the check that drops a knight
Seven seconds, at -7.7. This is the whole game.
The knight arrived on g4 with check and nothing defended it. The White queen was on a4 and the fourth rank in front of her was empty: b4 and c4 (you took the b-pawn on move 17, and the c-pawn had gone to c5 on move 26), d4 (you took it on move 20), then e4 and f4. 28.Qxg4, and the attack was over.
Nxf2 was the move, with the other knight, and the reason is that the same jump comes back two moves later for free: Nxf2 Qf4+ e5 Qxf6 Ng4+ Kh3 Nxf6, at -8.1. White checks, e5 blocks it, White takes the knight on f6, and then Ng4+ and Nxf6 win the queen back. Note that Nxf2 stops defending your queen on e1 and allows Qf4+, so it is not a safe-looking move; it is the move that counts to the end of the line.
You have done this before. Four seconds on 22...Rg8 against gayashan_desilva at -3.1 was the same shape, and it is worth noticing that both came when you were comfortable rather than under pressure.
What to drill next
- The position before 27...Ng4+: play it from here. Pass condition is beating the engine from this position three times running.
- Hanging piece puzzles. The theme is one sentence long, "a piece is undefended," and both of this game's mistakes are that sentence pointed at your own side.
- The position before 20...Rxd4: play it from here. Pass condition is finding the recapture without a hint and then playing ten more moves without leaving a piece undefended.
Before any check, ask whether they can just take it.