Hi wingly,
Between

and

, B already made some mistake
( hopefully other higher level folks can point out ).

Not the right way to play a ko threat --
this is a normal approach move to a 4-4: W can tenuki.
Instead, consider an attach: D17, or C16.
This way, your follow-up is continue with a second attach to his 4-4 stone.
This is very different from a double-approach to the 4-4...

...which you also missed. Even without any ko,

is soft.
At least double approach ( C14 or D14 ).

Look at the whole board:
Upper Left corner: W gets a
Better result than joseki,
thanks to

,

.
Lower left corner: your ko was a disaster for yourself;
you just gave W a HUGE corner.
wingly wrote:as far as I concern I open good
Up to here, Black gave up a lot to W.

K16.

Impossible.

You must cut W (K16).

Trying to be fancy. No need -- just connect at K16.
See
Toothpaste .

Very slow, very small -- almost a pass.

Again, small & slow; not looking at the whole board.

Again, small & slow; almost a pass.

,

Classic
Toothpaste .

Again, very slow & small -- almost a pass.
By now, White already has a huge lead; very difficult for Black.
wingly wrote:I feel like the game is equal but suddenly my opponent get lots of area
B had mistakes, slow and small moves ( as early as

..

),
and continued throughout the game to fall further and further behind.
White was ahead from early on; it was not sudden at all.

Very slow & small; also wrong direction: you didn't even block W (S13, instead).

119 Very small; almost a pass.
Biggest problems:
-

Very slow & small moves: you basically Passed more than 6 times.
-

Basic broken shapes.

See
Toothpaste .
-

Life-and-death ( Lower Left corner ).
- More advanced problems: ko threats (

,

).