1. Keep practicng: Most generic answer I know, but it's going to be the most helpful, simply put just practicing will help you predict your opponent's, know what cards to play, and how to deal with bad hands. Most likely you'll do better than a tier two or three deck that you have practiced with actively than a tier one deck you haven't practiced with. I have close too 200 games on with my Volcanion-EX deck. I know Gardevoir-GX and Garbodor decks are objectively are better decks, but I'll do better with Volcanion-EX because I've practiced and used the deck more, so just practice with the deck.
2. Think about you're previous games and see ways you could have improved: More often than not this will be mistakes you made, simply seeing those and accepting these will help you from making those same mistakes in the future. There's not much to this as it's really simple, but I'm suprised more people don't do this. What I do is while I'm in a match-up and I make a mistake I write down in a journal what that mistkae was and what I could have done diffrently, than after like ten games I look back at that journal and read it, it helps me improve, but that specific method might not help you.
3. Change the decklist to help your playstyle: Even though those decks have proven results, they might not work for you and you're play-styles, if you're having a tough time getting Gardevoir-GX out, it might be wise for you to add some more Diancies, Kirlia, and Rare Candys. Don't copy the best card for card, don't be afraid to make changes, it's your list, have fun with it.
4. Have a friend to help you: Personally I work on my lists with a guy who won Nationals a couple of years ago, but I understand most people don't have help of that caliber, but anyone who knows how to play the game semi-well can be helpul, bounce the list off each other and make changes based on what you find, then test with that list and repeat, doing this will help you perfect you're list, plus it makes the game more fun and more social.
5. Accept part of it might be you: This is going to be the hardest thing for most people to accept, but the issue might be you, the person who ran that list at Regionals probably put hours of testing and perfecting that list, but you most likely haven't put that much time into the deck, and there is a lot of RNG and luck in Pokemon, it wasn't just how good the list was, but that does help a lot. If we're willing to accept we might be the issue, this will help you improve.
I surely hope this helps you on your journey to mastering Gardevoir, best of wishes
@Seastrome!