Here's a game I played recently in which I thought I did ok, but lost.

I know I made a terrible foul up of the top left (black 59?? ), I certainly won't do that again, but are there any other major errors?

The big pushing battle from move 45 onwards looks weird, but I thought it was better for me because I built territory on an empty half of the board, while he built territory on the side with two small live groups of mine in. Was I right?

My invasion on the bottom was ridiculous, and began as a misconceived ko threat.

Actually, the more I look at it, the more terrible my play here looks...but any specific criticisms would be handy.

Any comments gratefully received as always.

A few ideas: (Edit: var at 43 -- thanks, topazg)

Sure, I'll give it a go. No promises:

15: Okay, so it looks like you've got a big enough area to live. I would take your sente and go ahead and play away. Specifically, I'd take C11. White really wants to extend from his new wall, so playing around halfway up will prevent him getting as much as he'd like. I recommend just north of the halfway point so that he's less able to shove you into his wall and attack you.
19: Same logic, though C11 is a bit dicier, after C6
33: T6 looks sente, may as well take it
35: L3 or L4 uses your wall and negates your opponent's.
51: Since you left your stone long enough that it's 3-1, I would just take the corner and then try to either reduce the left side or build your right to equal it.
65: This ko threat isn't that big because of the safety offered by the fact that he can capture at P1, giving safety to his bottom stones. It might be more of a threat, for example, to take N3 directly, so that you're threatening to capture those two. That said, I think you may not have enough threat to fight this ko, so perhaps you should have connected solidly before, sacrificing b16.
135: B6 is bigger locally, but H18 is probably biggest all-told.
169: B1 or C1, take your pick. Not worth risking. Additionally, the move you told defends a cut that doesn't work. If he cuts, you atari at M17 and it's connect-and-die. If you're defending your territory and don't have to, you're losing points. If you're not sure, it's fine, but it's worth checking after the game, so you improve for next time. Both sides did this sort of unneeded defense once or twice.

_________________
Someday I want to be strong enough to earn KGS[-].

Two things.

1. The bottom right corner is ko for a while.

2. You must cut m5 after white fills the ko. This is directly related to the life and death of the bottom right corner.

Excellent as usual, thanks to all three of you. Will study these comments today.

A few extra points from me:

15: Agree with Chew, although your move is double sente I think, so not small either.
19: Agree with Chew again. I'd always be tempted with C11 here - with the ability to make a two space jump on either side, it effectively breaks up the left. This approach move just asks White to pincer tightly and make a large framework.
33 and 35: Totally agree with Ed
37: Huge, and worth appreciating. These hanes when you have a wall impact the balance of power nicely over a large area of the board, and remove a lot of bad aji.
43: Disagree with Ed -- sgf edited, in agreement now --
45: Very big, but B16 slide and C11 extension feels bigger now. This is certainly a huge point though.
49: Torn on Ed's point here. You've got a big open side, and as White has R11 I think there's too much aji to play Ed's move. Your move is safer, and only a few points less valuable. K6 is also playable, and feels more severe than the hane.
51: I agree with Ed here.
59: Definitely agree with Ed's move here.
65: Disaster of a threat You followed it up by praying you could live small, without killing anything! At least play a move like T6, which is sente and threatens to kill big.
99: Good, and hugely important area, but maybe just L17. He's as thick as you, and a pincer could make life very hard, so playing it safe is often good.

topazg, thanks about 43 -- B cannot double hane; I fixed the SGF.
Not sure about B connecting at O6; B nobi N7 in the game seems OK.

Yeah, that makes sense, I bounced between the two for a while. I opted for the connection because it seems to do more to the rest of the bad cutting aji around there. I'd want to open up the chance for moves like M8 later if the top made it look more interesting. However, the nobi can't be bad.

Since no one else mentioned it - you can also take your time! Go is fun. There's no real sense in using your main time faster than you would have to play your byo-yomi periods, right?