I double-down on anything 10 or higher and win 90% of my hands.

I hope by that you mean 10 or 11. You should never double down on any blackjack hand over 11, unless it's soft (includes an Ace).
The odds also favor doubling down on hands totalling 7-9 when the dealer is showing a 4, 5, or 6. A soft 17 always warrants doubling down against those cards.
I realize that Luck stacks the odds in your favor in this game, but you still maximize your odds of winning by playing blackjack correctly. And, if you ever happen to find yourself at a RL blackjack table, you want to know how to play correctly. People around you will get incredibly angry if you play your cards wrong and end up helping the dealer.