Oh man there are so many things I want to go over. Because I have no clue what or how much you're doing wrong I'll try to cover all the bases.
-The first thing I want to say is that netdecking top lists doesn't always work. Those decks did well because they were prepared for that event, and now that those decks are on a lot more players' radars, they're going to be more prepared in the following event for it. This is how drastic meta swings can happen, like how Pika utterly dominated the 3rd Limitless Qualifier and still performed decently in Qualifier 4, but fell short of how good it was in the previous tournament. You need to know what people are playing at an event in order to do well. Because if you think the meta is going to be the exact same as before and you just play the winning deck, you're going to do a lot worse if the meta shifts to beat that deck, which is usually what happens. Another thing is that you may not be playing the deck optimally. When somebody does well at a large event, it's because they spent many many hours testing matchups, finding what lines of consistency they like, and preparing for decks they expect to do well. If you aren't taking ALL of these same steps, then you just aren't putting in the work you need to do well with the deck. Testing weird situations, learning what your opponents want to do against your deck, and finding optimal play lines are all things you should be doing with every deck you want to play in a serious setting. Another thing is that you could be missing something you didn't even think of, which is why whenever I pick up a deck I try to watch skilled players play against other skilled players so I can learn the deck without going in completely blind.
-One of the most important things in Pokemon is proper sequencing. You want to maximize your odds of hitting a certain card if you need it off of like a Stellar Wish, a Dedechange, a Marnie, etc. Thinning your deck of shit you don't want to see means you don't draw into it instead of the card you actually need. Like if you need to Boss to win this turn or the following turn, you Quick Ball into whatever Mons you have left in deck, you Dedechange to lower the amount of cards in deck so you Stellar Wish 5 out 15 instead of 5 out of 21 (or instead of 5/22 if you used the Quick Ball and took something). There also might be times where you break proper sequencing because it could influence a decision you make. This happens a lot when playing Welder decks because if you Quick Ball for an attacker and miss Welder off of Stellar Wish you just pass. So sometimes you Stellar Wish first, and if it's not a card you need, you can Quick Ball for Dede to see if you get any more options.
-Prize checking is also another extremely important skill in Pokemon. You need to know your 60 off the top of your head, because you need to check quickly and accurately. Prizing can mess with sequencing because certain odds get lowered when cards are prized. And if your last/only copy of something is prized and you aren't paying attention, you're going to make a very bad play based on information you have access to but didn't bother using. And almost every single time that happens, your opponent is going to capitalize on it.
-All three of these skills are very important and all need to be used in conjunction with each other. You need to know your deck to win, and you need it to check prizes. You need to check prizes to play correctly and make sure you don't need to change your regular sequencing, and you need to know how to sequence to play your deck optimally.
-That is the basics of what you could be doing wrong. However, other factors can interfere with your results.
-Who are you playing/testing with? If you're playing against incompetent players who don't pilot their decks correctly, you're going to win against them. But if you take that game as an example of how to win and you play against a highly skilled player who already knows everything I'm telling you, then the result is not going to be the same. This is why top players associate themselves with teams. Getting other really good players that think differently gives so much leeway to winning matches when you can test every outcome of a matchup, and even settle a lot faster what the optimal way to build/play a deck is. Not every good player is well known, so it's not like you need to spend money on Azul's coaching to learn how to play Combo Zacian. If you know people, you can find reliable testing groups. Everybody on my team has a completely different play style and our accomplishments range from very few regional placements to multiple good Limitless Qualifier finishes. We put in the testing for the few events we play in and it shows because I think my teammates are very good at the game and I trust their skill enough that I can recreate the results of our games against good players (and I did at Collinsville).
-Are you just getting unlucky? You could be playing a normally consistent deck and you just don't draw well with it. Best of 1 scenarios don't do any favors to unlucky games, so if you're getting beaten by bad players on ladder it could just be from bad luck. I played ADPZ in Q3 and started 4-0 and then ended up 6-5. I played almost the same 60 in Q4 and ended 14-6, bubbling top 32. I just had better luck in the second event I played with the deck. Top players get bad luck too. Whether it's bricking, horrible prizing or just hitting really bad matchups it happens to everyone. Henry Brand missed day 2 at OCIC with Ultimate Mew3 before winning a regional with it.
-Lastly, are you playing worse because you're burnt out? If you're getting frustrated because you're losing, you are always going to play worse. Stepping away for a few days is a great option to refresh your brain and trust me when I say you'll magically play better when you actually want to play. Focusing on "improving" isn't focusing on playing the game, and will lead to overthinking really simple decisions.
Your results are going to reflect how serious you are about getting better at the game. I think I covered all the bases, but if you have any specific questions I'd be more than happy to answer (assuming I can).
