xoxox wrote:I still don't understand why

is a small move? Doesn't that secure territory more than 2 points?
(schawipp already answered while I was typing the following paragraph):

is small because your group is already quite safe, White needs several moves before threatening anything. You are just adding about 6 points to an already strong group, and you are becoming overconcentrated (too many black stones in a small area).
A guiding principle is that corners are big, so most of the time, approaching a corner (C6, F3) or making a corner enclosure (C14, F17) is a big play. Another possibility is D10 since it interacts loosely with D16 and prevents White from playing there. Or perhaps play around H3 and try to live (this is basically what you did around move 183 but you died because there were already too many white stones; you would have had better chances of success by invading earlier).
Some of these moves are probably not optimal at all, but I am pretty confident that they are better than your move
xoxox wrote:
Any suggestions about middle game play?
After

White could have captured a stone (e.g. N18, O17, L18).

gives you bad shape because

crossed your keima N14-O16.

is bad because you lost sente. You already have a bamboo joint (L8, L9, N8, N9) so White cannot cut through it.

is a small move because it is gote (White doesn't really need to answer and could take a big point elsewhere).
Move 101: if your idea is to kill the D15 stone or at least to save your C15 stone, then C14 gives you a better shape (whereas the atari at D14 leaves a cutting point at C14).
Move 137 is often bad shape because it invites your opponent to
cut your keima.
Move 167: what are you afraid of?