Here's a game I recently played. I'm Black, and about halfway through I think I'm behind by a lot, but at the end of the game I was expecting a 20-30 point difference. It ended up being only 8.5 points that I lost by.

Here's the game:

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My plan to become an SDK is here.

My recommendation is doing a bit of tsumego. Your main problem is that you both missread a lot. This game had several missed ataris. And several more situations where looking a few moves ahead would have avoided large captures.

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Sanity is for the weak.

16: Good style.

26: Try S19. It kills.

32: You're making connections.
You still came out of this corner better than white.

34: Corners first, sides next. P4 or P3.

38: Not bad, but not urgent. He has a ladder breaker and your upper right wall could collapse. Fix your connection with L15. Then you are threatening E17 and K17 etc.

46: Better to tenuki than make a move this ugly.

48: Before this move, you had 4 liberties; now you have 3.

64: Good idea, maybe a tad too close to your own wall.

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But: always capture in a net rather than a ladder, where possible. Connecting a group with a ladder when you could have had a net is like making a ko when your group should have just been alive.

86: This is 5 points. In gote. It does not connect anything, nor disconnect anything, nor threaten to kill anything.
You have much bigger moves available like L15 or E17 or R6.

92: Throw in at A3, then play B6, A7, and A5.

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Why not S15?

Doesn't it work? or maybe it's preferable to leave a weak group to tempt your opponent?

To channel Ed Lee, it seems you spent two seconds on move . Consistently take your time and you won't make many mistakes like that.

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Occupy Babel!

Yes, S15 is clearly better.
No, L15 is slack. N16 can be captured in a net: N15, M14, N14, N13, O14, O13, P14, Q13.

A3, then B5. Instead, B should just play B6 straight away. B6, C5, A7, A8.

Also, I'd like to throw out there that you had a chance to win the game as late as move 280, merely 10 moves before scoring. W's 279 was incorrect and should have been R3. Now you can play S3. Regardless of whether W captures Q3 or blocks at S4, you gain enough to win you the game.