Can someone help me to understand the exact point in which I made the wrong move in the following game? It would be very appreciated and I believe I would learn a lot.

20...hxg5? was the decisive error. Should have kept the position closed with 20...h5.

I think that e6 in the opening is a bit passive, I would probably move c5 there, but of course it is not as though e6 is terrible. I think that you should definitely consider trying to trade off that light squared bishops with Ba6 somehow (although that is not possible after 5.c3 due to 5...Ba6 6. Bxa6 Nxa6 7.Qa4+). Mabye it was a good idea to play a5 and Ba6 at some point in the late-opening/early middlegame.

Your move h6 was unnecessary and weakened your kingside. After that you play passively and seem to have no plan against what he is obviously going to do.

When you are playing against the stonewall and openings like this, it is very important to come up with a plan pretty early, for example: trading off the light squared bishops, or landing a knight on e4 and playing f6-e5.