Good opening leads to worse position, when you have no good plan

This is a game I played in the second round of the Polish Team Championship (2nd league).

I was surprised with the opening and in the 5th move I was out of any preparation. After some development which seemed natural to me, I got a good position after the 19th move. 

I wasn't sure how to proceed the best way, but I kept the good position. Unfortunately, my opponent outplayed me in the last 10 moves and I got a lost position.