direction of play and timing issues

[Erasmas]

In this game I played against a much stronger opponent without handicap. It felt like one of my best games and it was really close for a long time, before I finally messed up too much. My opponent afterwards told me that I missed important endgame plays, which could have gotten me some 15 points. I have to admit I did not spend much time learning those, because my games seldom last that long. Instead I'm trying to improve my middle game-in particular regarding the direction of play.

I'm open to any remarks, but these are the questions I wonder about from my point of view:

• Was it ok to leave the top-left corner open for so long? White admitted that 20 was a big mistake. 22 was urgent, so I felt 23 was finally the time to grab the corner.
• L17: is this too greedy? Also is there a follow-up around O17? I couldn't read anything good there, but my gut tells me it should exist.
• black 51: Again direction of play.. I felt this was needed, and didn't want to get committed to the bottom two stones. I guess though I should have followed up on 51 instead of the K3 play?
• black 83: I thought this was a huge move. My plan was to pressure the cut off white group, in order to develop strength in the center, as the succeeding reduction should ensure my win. It worked well for a few moves, but then I was told that O14 should have been at M12. I was hoping that O14 allows me to keep up the pressure for longer.. Is this simply a case of my center group being too weak for this sort of attack?

[TIM82]

Some ideas, as usual, take with a grain of salt.

[EdLee]