There's really only one thing that separates an average player and a great player, be that in any fighting game or even in any fight IRL, and that's muscle memory. You might know about all about the matchups, what to do in every situation you're in; if you think about all that, you'll only ever be average at best. It's when all that knowledge starts being acted on subconsciously that you'll be great.
Humans evolved to use reason and logic, not instinct. That's all well and good, really, except when it comes to fighting. If, for example, someone whiffs a counter and you think "okay, he whiffed a counter, just gonna smash him when the counter frames are over and he's vulnerable", you won't get it. By the time it takes your brain to analyse the situation, deduce the correct response then send electrical impulses to your hands to press the buttons for the smash, the opportunity's going to be over before you can even react in most cases. However, if you don't think about it and just do it because you've done that a thousand times already, you'll be fast enough to get it. This frees up juice to think about the larger picture, read the opponent and come up with an overarching strategy because your brain isn't preoccupied with the minute commands, this is done on an instinctual level.
All the computations that accompany logic must be done beforehand and this must be accessed subconsciously in games (or fights) if you want any real success. One day you know how to do, will access that in your brain and try to do it, which takes some time, and another, it'll just become instinctual for you to do, and the impulses will just instantly shoot out to your hands.
I used to be pretty bad at Judo because I used to use my brain consciously to analyze balance, distance, position, etc. By the time I saw an opportunity and reacted to it, it was too late, I'd get countered or dodged and I'd get frustrated. On defense, it was even worse, because my brain panicked and took a long while to get what throw I was being attacked with and what to do to counter or dodge it. One day, I did the best throw I had ever done until then, randomly, just like that. We were both on our feet one moment, the next he's flat on his back with me in a scarf hold. I don't remember thinking about the throw, it was one I never consciously used, it really just shot out. Ever since my brain "unlocked" and I started relying on subconscious analysis of my sensory perception instead of using up juice from the main processor, I have had success in Judo and suddenly it became much funner to practice. I knew throws, counters, ways to unbalance, combinations, etc, but I didn't have to analyze which to use when anymore, I just trusted my senses implicitly and did what they told me I should do. Unfortunately, I haven't had that happen with Smash yet. Anyway, my point is that you'll suck hard and long, but one day you'll just feel something "unlock" in your head, as if you were in "the zone" and then you'll thrash people left and right, just like that. Congratulations, you now have great muscle memory.
This is why often top-level players can't tell you what they're doing and how. They're not thinking about it, their processes are not in the same place as yours. There's only one thing you can do to acquire that, and it's practice. Someday it'll unlock.