Hi Oca
It is very useful that you have provided comments, so that we can understand what you are thinking. There are many things to say about this game and the thinking behind it. I would say that the most important issue is to
1. Understand the function of stones
At
you say "but I wanted to strengthen L6". However, after the capture of your group in the centre, the stone at L6 is a useless waste of a stone, thrown against a strong black position. You shouldn't try saving it, not connect it or strengthen it, not before the late late endgame where the 2 points it represents may be the largest value.
Compare this stone with your stone at E16 which is contesting the large open top left area. That one needs strengthening, for example by attaching to the opposite black stone at C17. This stone is worth about 20 points. You understood this well at move
. The idea behind this move is good.
2. Keep track of liberties
Near the end you lose a couple of groups by lack of liberties. This is an important point to pay attention to, because you throw away victories that way. We should always be aware that near the end liberties become scarcer and our territories may need connecting or eye making moves.
3. Stay away from strong groups
In the early middle game, at
you try attacking Black strongest group. There's no need to do so and it is pointless. You should accept that some opponent groups will survive and this will be especially true for their strong groups. At
you try to "live by ko" but there is no ko in miles around. The white stones are captured, as was to be expected after they attempted to attack a strong, connected black group with eyespace. In fact Black could have attacked these stones on a much larger scale, but that is another matter.
So, to recover:
1. understand the function of stones: useless stones, important stones in contested area, important cutting stones, stones with some potential remaining
2. keep track of your liberties towards the end (as throughout the game)
3. stay away from strong groups
If you can focus on these things to improve, you will improve on the whole by a lot. In terms of increasing your victory %, n°2 is the most important one. In terms of your growth n° 1 is most important while n°3 will give you long term growth.
Must be Q6, no choice.
Can try Q3 to make White over-concentrated, and settle your stone.
You can try S5 to fight. Otherwise the result will be poor.
Must be double hane at G2 to make sense of your way of playing.
Not a good idea.
Interesting.
But this doesn't work ...
...because the other atari is better than this one, given the cutting point at E3. Spot the shunt.
Should probably just be N8, though Black also has to worry about getting cut. As it is, Black's shape falls apart.
)
which should be at "a"
is bit annoying 
... that give me lot's of idea !
