Life In 19x19

I absolutely killed my opponent. I still want my mistakes pointed out. I felt I played well, overall. I'm white.

looks bad, it gets cut and result in a traverse keima + traverse tobi.
feels premature, the side is still open, and approach is possible, as well as a wedge on the side.
would be better at N4
?? What was that ? And as EdLee would point out, you only took 3 seconds to play this no brainer.
S4 first too keep the corner.
don't attach to weak stones. You got hurt in the process.

But sorry to say that, the biggest mistake was to select this game for a review. A one where you were outplayed would be more instructive. Here, you didn't get punished for most of your mistakes.

Edit: I watched your previous game as well, hope you don't mind. It was much better, and I can see a more general mistake : you like to push from behind.
, , , , 104, 106, 108, ...

There are other mistakes and small moves, but this looks like the easier to fix now, and it will helps to pay attention to that. Don't push from behind, and don't drive white in your territory.

13: Too close to his strength, and leaves a gap behind which he could occupy with C14 leaving your stone stranded. If you want to extend from D16, best is probably C14 or C13.

21: A kiema in heavy traffic is too weak. It you want to move out like that, F5 is way better.

27: This is premature. Usually the lone 3-3 invasion is made when he has stones on both wings of the 4-4, thereby endangering any approach move. R6 is much larger.

37: Shorting yourself on liberties. If you really want to make something of N3, extend to N4.

45: Slow, small, and gote. R6 probably gets a big corner for you.

49: Wreck his shape with S4,

51: Again, S4.

52: Note that when he finally controls S4, he starts to look alive.

As a general rule, when your opponent makes an empty triangle, the points 'a' and 'b' are the vital points. You need them to attack; he needs them to make an ugly shape into a slightly less ugly shape:

Click Here To Show Diagram Code

[go]$$B
$$. . . . . . . .
$$. . . a . . . .
$$. . O X c . . .
$$. . O X X b . .
$$. . . O O . . .
$$. . . . . . . .[/go]

BTW, this is also usually true if you have a stone at 'c'.

65: T7 starts a ko for his life.

67: Good shape.

71: Another example of 'when not to invade'.
You have strong shape, he has gaps. The invasion encourages him to strengthen himself. Q14 or R14 would exploit his weaknesses.

85: P15 is huge. ( note the similarity to 37 )

89: Big, but slow. N16 chops him up. G10 is big too.

91: Contact moves are often defender's moves. If you want to kill this stone, stay back one space with E14.
Also, you might play B10 first.

140: This is small. Let him have it if he wants it. You can make more with O16 or H3 or B3.

233: It's good that you saw that you don't need to connect (unlike H9), but tenuki is premature. Black isn't fully alive yet: at a minimum, you can rack up a ton of points in sente, but after a quick look I think you can kill. o15 pops out an eye in sente (threatens to cut at o14), s10 pops out another eye in gote, and then o11 and p11 are miai for the last eye (after w p11, o10 and o9 are miai).

I'm probably wrong about the status of this group, but I think the general point is worth keeping in mind. The middle game is about the life and death of groups; the end game is about establishing the exact borders between living groups.¹ But the biggest endgame points are grabbed in an intermediate phase, where you are establishing the exact borders in a way that threatens to kill or destabilize enemy groups. Here, you jumped straight from the middle game to the bitter end of the endgame.

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