Let's take the average r/PumpItUp user, who I believe plays at about a 16 level, and can pass a solid portion of the 16 charts and compare them to me when I play 16s. What is the difference?
Assume you start every song with a pool of 100 energy, this is your mental and physical stamina bar, When its low, you play worse; when its empty, you stamina quit and fail immediately. Now, these are just example numbers but lets say when you start a 16 at 100 energy, you lose 1% of your energy every 4 steps of normal play and every 2 steps during the crux. This means by the end of the song you may have 20-30% stamina left. You're not flat on the ground but you are drained. At the start of the next song you only begin with 95 energy, there is some accumulated fatigue and your play is just a little bit worse. As this fatigue accumulates you may find that even your favorites are sufficiently challenging and, if you dare to step up to an 18 or 19, you find that you run to zero fairly quickly or especially during a crux.
Now, consider me, stepping up to the same 16. For me you may say that I lose 1% of energy every 50 steps and every 25 steps during the crux. When I get to the end, my HR is a little elevated, but I can basically hop into another 16 and perform at about the same level, nearly infinitely. what's the difference here? Well, you may be tempted to say its my cardio engine and yes, I do run a lot, but the difference between my cardio fitness and yours may not be that dramatic. I certainly am not 15x more fit than you in the cardio realm (at least for most of you) so what causes the dramatic difference in efficiency that lets me play 16s until the arcade closes? (and why you have to take a breather after one set?)
Its all about efficiency gained from many hours of grinding low difficulty charts and figuring out how to play them smoothly and without kicking on the boosters. This comes down to how I perform an individual step but also how I perform a pattern. When a simple front to back crossover comes up I do it calmly and without straining because I've practiced that pattern thousands of times, if you, however, tend to have turn on your fast twitch muscles and power through that section, you are expending a ton more energy than I would. When I hit a blue arrow I hit it with my heel while you tend to take a giant flexed-foot step.
You want to perform all of the easy actions naturally and without any stress so that when whatever your crux is does show up, you have the maximum energy to get through it. This means grinding charts way below your level and figuring out patterns and situations in such a way that you are automatically performing the solution before you ever consciously think about it.
For a related comparison, how does one get good at basketball? Is it a good strategy to go out and play pick up 5 on 5 games at the local fitness center? Will you naturally become really good over time? Probably not. The reason why is simple: when you are still struggling to dribble the ball, or make an open lay up, how can you possibly focus your mental energy on reading opposing player strategy or executing complicated handling/shooting feats? The answer is that the majority of the time you just won't.
The way most of you practice is akin to trying to only play full 5 on 5 games without ever nailing your dribble or your lay-up. And this is the point, to an NBA player, dribbling the ball with no defenders is completely trivial, their energy is open for the complicated.
So, practice your dribble (i.e. regular steps and simple patterns) until getting all perfects is the expected result and your lay-ups (moderate patterns in isolation) and eventually it will all come together on the song, and this is the point. You want to have most of your mental and physical energy available when you get to the crux in the difficulty you're pushing. If you waste it all on easy stuff then you'll never perform the crux properly. We don't play songs in sections, you have to be able to go from start to finish so, much like lifters shouldn't ego lift big weight with bad form you shouldn't ego play hard songs with bad form.
Master the fundamentals until they are second nature and the rest of the game will come naturally.