I'd boil all this down to something two-fold:
1. It sounds like you mostly lose by getting nailed by your opponent's shadow, less often by his freeps beating you to the end. So, the decisions you are making are mostly having your freeps double-move when they shouldn't have, and to a lesser degree are allowing your opponent's freeps to double-move when they either should have been intimidated to stop, or gotten destroyed on the double. Which leads to...
2. Predicting what your opponent can/will do. This goes to what the guys above me said: Look at what your opponent is playing, try to figure out what kind of deck you're up against, and play accordingly. What kind of minions has he played on you so far? What sorts of strategies does that deck use?
You say you think you've got deck-making down. In that case, I'd suggest copying a deck that has actually won championships, and play with that for a while. This removes your deck-building skills from the equation (you KNOW it's a good deck), and allows you to concentrate solely on your playing style, on your tactical decisions. If you suddenly start winning every game, maybe your deck building skills aren't as good as you thought. If you are still losing regularly, you know the issue really is something to do with the way you play, the decisions you make during the game.
I'd also ask: When the game is over, how many cards are left in your deck? Because if you're discarding, drawing, and playing properly, you should be just about out of cards by the
end of the game. Not completely out, but I'd say 85-90% out. If you've got more cards left than that, your decks may be too big, or you may not be playing cards/discarding fast enough.
The people you regularly play against can have a big impact too. Different groups tend towards different favorite strategies. A deck that I might regularly beat my friends with, might utterly fail when I play it against a group of strangers. Because I'm not used to the way they play, or the strategies they prefer, my playing may not be as effective.