You sometimes don't finish your corners. This costs you a lot.
Corners are, as you probably know, the most valuable parts of the board - particularly early in the game. They are frequently contested.
Often, one player gets the inside, and the other gets the outside. The inside is territory, the outside is influence. I don't see a clear recognition of this in your play.
For example, your opponent gets the outside, and you should immediately recognize that your destiny is to take the inside - or get almost nothing. But you tenuki.
Example 1 from the first game:
- Click Here To Show Diagram Code
[go]$$c
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . X . . . . . , . . . . . X . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . , . . . . . , . . . . . , . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . 6 . O a . . |
$$ | . . . , O . . . . , . . . . . , . . . |
$$ | . . . . . . . . . . . . . 7 . X . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------[/go]
Move 6 is a very influence-oriented move. ( It is also rather dubious, only good under the most rare circumstances )
When he plays it, you should remember that if he gets influence, you should get territory or vice versa. And immediate thereafter you should recognize that your direction of play is to take corner territory.
So you should play 'a' without thinking. Move 7 as you played it is a move oriented toward the side. But corners are bigger than sides.
FWIW, he doesn't understand this either. When you fail to get the corner territory, he should try to get it himself by playing 8 like this:
- Click Here To Show Diagram Code
[go]$$c
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . 6 . O . . . |
$$ | . . . , O . . . . , . . . . . , . . . |
$$ | . . . . . . . . . . . . . 7 . X 8 . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------[/go]
Example 2 also from the first game:
He has influence with the marked stones, so your reflex should be to take the territory with 'a'. But you played 'b'.

- Click Here To Show Diagram Code
[go]$$c
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . W X a . . . |
$$ | . . . X . . . . . , . . . W O X . . . |
$$ | . . . . . . . . . . . . . . X b . . . |
$$ | . . . . . . . . . . . . . . . . X . . |
$$ | . . . . . . . . . . . . . . W . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . , . . . . . , . . . . . , . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . O . O . . . |
$$ | . . . , O . . . . , . . . . . , . . . |
$$ | . . . . . . . . . . . . . X . X . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------[/go]
Example 3: A few moves later, he makes a mistake, and you still have a chance to get the corner territory - in sente! - but you tenuki.

- Click Here To Show Diagram Code
[go]$$Wc
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . 3 c . . |
$$ | . . . . . . . . . . . . . O X 1 2 . . |
$$ | . . . X . . . . . , . . . O O X . . . |
$$ | . . . . . . . . . . . . . . X X . . . |
$$ | . . . . . . . . . . . . . . . . X . . |
$$ | . . . . . . . . . . . . . . O . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . , . . . . . , . . . . . , . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . O . O . . . |
$$ | . . . , O . . . . , . . . . . , . . . |
$$ | . . . . . . . . . . . . . X . X . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------[/go]
He should have captured at 3 instead of descending, which means that you can get corner territory with 'c', and do it in sente. But you tenuki. Consequently, he plays 'c' and gets some territory AND outside influence - in a corner where you played first.
Example 4: still from the first game.
With 23, you chose the outside, and your next seven moves are influence oriented. It is not bad play, and is certainly consistent. ( One could quibble, noting that 25 would be stronger at D18 and 27 at E17, but such conservative play may be excusable since he has all the ladder breakers in the lower right corner )
You play eight consistent moves getting influence while he gets territory, and then when a solid outside wall is just within your grasp with D14 at move 39, you tenuki.

- Click Here To Show Diagram Code
[go]$Bc Position at move 37
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . O O . . |
$$ | . . O O . . . . . . . . . O X O X X . |
$$ | . . O X . . . . . , . . . O O X . . . |
$$ | . b X X . . . . . . . . . X X X . . . |
$$ | . . W a . . . . . . . . . . . . X . . |
$$ | . . O X . . . . . . . . . . O . . . . |
$$ | . . O X . . . . . . . . . . . . . . . |
$$ | . . O X . . . . . . . . . . . . . . . |
$$ | . O X , . . . . . , . . . . . , . . . |
$$ | . O X . . . . . . . . . . . . . . . . |
$$ | . . X . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . O . O . . . |
$$ | . . . , O . . . . , . . . . . , . . . |
$$ | . . . . . . . . . . . . . X . X . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------[/go]
The only consistent move here is 'a'. ( Although an attacking player might play 'b'. Even then, it implies a recognition that white's goal is territory, and it aims to disrupt that territory in trade for having one's own influence disrupted. )
With a good sense of the direction of play, it should end up like this:

- Click Here To Show Diagram Code
[go]$$Bc
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . O O . . |
$$ | . . O O 3 . . . . . . . . O X O X X . |
$$ | . . O X . . . . . , . . . O O X . . . |
$$ | . 2 X X . . . . . . . . . X X X . . . |
$$ | . . W 1 . . . . . . . . . . . . X . . |
$$ | . . O X . . . . . . . . . . O . . . . |
$$ | . . O X . . . . . . . . . . . . . . . |
$$ | . . O X . . . . . . . . . . . . . . . |
$$ | . O X , . . . . . , . . . . . , . . . |
$$ | . O X . . . . . . . . . . . . . . . . |
$$ | . . X . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . O . O . . . |
$$ | . . . , O . . . . , . . . . . , . . . |
$$ | . . . . . . . . . . . . . X . X . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------[/go]
In summary: Remember that corners are big, and if you want to win, you must fight for your share. Often the division is influence for one player and territory for the other. If he gets one of those, you must make sure that you get the other.