11: You are safe in the upper left, so the biggest move is probably the open corner. However, if you do want to build the top while he builds the side, keep in mind the proverb 'a play at the border of two moyos counts twice'. E14 is sente for you ( threatens C13 ) and F14 is semi-sente for him. If you play K17 here, interpose E14 first.
21: This move feels small and passive and in the wrong direction. It is the kind of move that you should make when you are surrounded and have to make space. It does not feel right when you have access to the middle and eye space with R2.
Not only is it the wrong direction, but after the natural reply at Q9, you lose sente.
I'm not sure how to handle the right side, but this can't be it. Maybe Q8?
27: An interesting move. It does follow the proverb of attaching to strong stones instead of weak ones, but it is hard to find a good continuation after the natural reply of C5.
If you want to attach to facilitate an invasion, K3 would be better, for it offers the possibility of crosscuts, and - if you can play R2 at the opportune time - you might actually be able to make threats to kill.
29: This is just too weak. You are compelling him to strengthen himself and getting almost nothing in return.
31: Something that resembles sabaki - a good choice in a situation that is otherwise headed resolutely downhill.
33: Much better.
35: Ooops! It was looking so promising for a few moves there.
Now you are encouraging him to connect. K3 would have separated him.
41: This is an attacker's move, usually used to make the defender heavy and shapeless. But, because his natural reply makes him easily connected to K3 and its associated eyespace, it does you no good. He becomes strong, not heavy.
A good example of a defender's move would be H7.
FWIW, the best strategy here would be to ask yourself what those three stones of yours are worth to you.
They cut nothing, they protect nothing, they make no territory: they are absolutely worthless to you. Abandon them, cut your losses, and move on to something with greater potential for you. O10 might be fun.
43: Again: you are investing your effort in worthless stones.
47&49: Much better!
53: K12 is huge. You need not worry about O4. If he tries too hard there, you will become strong around N9, and then you will be able to kill everything of his on the right side.
64: Sigh. He is being too prudent. Now you probably cannot hurt his right side. It is all over but the tagging of toes.
In conclusion: set up the final position on a board and look at your five stones in the lower left. They never accomplished anything but to make him stronger. An obvious side effect of that strength is the death of the five stones on the left side.
Those ten stones, the territory under them, and the territory near them are probably enough to make a 50 point difference. You could have beaten a player 6 stones stronger than you if you invaded/reduced differently.
Look at your move 31. It was a fine play, an example of good sabaki to invade or reduce his territory. If you play more moves like that, you will be 2K soon.
