It is partially the way you build your deck, but also understanding how to use that deck to your advantage. You gotta keep playing in order to improve. Synergies matter as well. Not just the color of the cards, but the effects of the cards. You also need to balance your deck properly. I have seen so many good decks lose, because their opponent was quicker at building momentum when their strategy was solid in general.

But sorry to hear that otherwise.