Hey PP,
So this is kind of a complex question that might be hard to answer, but I'll do my best to communicate it. I feel like my game against the actually good players is very much based around the things that work against people below a certain level on the tier list. For example, if I'm fighting against a really good Luigi, I can generally get by with a simple gameplan of things like "stop his movement with lasers, use ftilt and spaced fading dair/bair to stuff ground approaches, and shark his fall to the ground by waiting for him to get antsy and then whiff punishing his aerial." It's simple, and because of the limitations of lower tiers, that simple gameplan takes me quite a long way against the players of higher tier characters that aren't player a higher-level game.
So being a little more specific, against Fox I feel like I annihilate a lot of mid-level players. I feel like I have a good mixup game of approach vs movement mixups vs baits vs lasers, etc, and my punish game is generally good enough to get at least one ZTD per game. But then, when I sit down to play against an actually good fox, such as when I sat down a while ago to play Crush or DJN, or when playing in tournament against a mid-level fox who is completely patient and content with just biding their time, I just get completely exposed. Specifically against DJN, when I asked him how he was beating me so consistently he told me I was getting most of my openings off of crouch cancel and bair, and if he just played around those I didn't have any way to open him up. So I started looking for that more closely, and I noticed that he was right, that a lot of my openings even against mid-level players were coming from those tools...but the rest of my kit wasn't working against high level players and I suspect it's because they were way too big of commitments and could be reacted to from far enough away that they'd never land.
This is the case in most of my top tier matchups; it's actually kind of messed my game up because I'm looking at every time I make a big commitment and I don't know how to make plays in a low commitment way. I know how to like, keep people out or just poke. So if I've got a marth in the corner, e.g., I know how to just laser ftilt or do platform mixups into bairs if he tries taking stage, but if he's content with sitting there (hypothetically) the whole game, I have no idea how to safely commit to making a play without giving up center, making a big commitment like an overshot aerial, or raw smash attacks (which I do way more than I probably should against high level players). If I try low commitment things, I lose my mixup game and it's much easier for them to dash dance/whiff punish or something. And what's more, I get extremely nervous when I'm forced to play a low-commitment-in-all-cases gameplan, because a) it's not what i've been practicing for years, and b) I know that the tiniest mistake is what will get me opened up and killed. I feel less nervous when I do a dashing dair into the corner against Marth, who rolls in and kills me for it, because I can shake that off as a really dumb mistake I shouldn't have made. But when I'm trying to do run in fadeback dair at the very very edge of his dashing range to be as low commitment as possible, it feels like if I'm not pixel perfect I will die for no possible return, which makes me play more nervous and more likely to fall back into bad habits.
This is all a very long-winded way of saying that my problem at the moment is a couple layers deep. 1) I only have a couple reliable tools in my pocket for the top tier matchups (specifically fox, falco, and marth, although the issue extends to some falcons and peaches too). 2) The reason I don't have more tools than that is because my tools against mid-level players are higher in commitment and work due to their being less technically proficient and tactically skilled. 3) I don't know what to look to in order to build up an arsenal of low-commitment options capable of both keeping me safe and making plays. 4) As a result of not knowing what to do, I fall back into a very basic pattern of behaviors/mixups. 5) I then can get picked apart, and that + my ignorance leads to frustration and nervousness.
My apologies for the essay, but hopefully there's enough to chew on here that you might be able to offer advice on what to look to for improvement or how to fill in this gap in my play. Thanks.