How could I screw this up ?

Hi everyone,

So I just resigned this game and thought "How could I screw it up ?".

Replaying it I think that 10. f6 should have better been played j3 or i4.
19. d9 is also problematic but would even have been so in c8.

In one word... the both groups of my center where connected to each other but their connections with sides were doubtful.

These are my thoughts, please tell me yours.

Do you mean 18.d9? What's wrong with 18.c8? It restores the connection to the left edge. And you seem to have a climbing on the right.
On 19.b9 you can move 20.d9 .