I can sympathize where you are coming from. Performance anxiety is common in these types of games and that's a whole other topic. But moreso than some other currently top end MMOs, FFXIV is about choreography. While there are some common markers and recycled mechanics, there's frequently an expectation that you know precisely what to do in a particular boss fight even if there aren't always easy to interpret telegraphs. As you go from dungeons and normal trials to EX trials to Savage Raiding, the choreography becomes more noticeable, longer, and more complex. The pinnacle of this process is Ultimate Raiding.
Even if we nix all but dungeons and remove most of that choreography, as mentioned some telegraphs aren't clear in their meaning on their own. Oh the boss's eyes flashed blue... what to do? Oh now they've flashed purple? Or a boss raises a hand... in one case it's sign of an impending tank buster and in many others it means stand on the boss's other side to avoid a massive AoE. Hey look the boss' staff got longer... does that mean get closer or does it being shorter mean get closer? Sometimes things appearing over your head that aren't the common stack marker also mean huddle with other players, though most of the time they mean run away and drop an attack away from other people. Stuff like this is littered throughout group content. And... there's a LOT of group content.
Some people will reflexively tell you to watch guides, which is fine and worthwhile, unless you are doing random roulettes. If you've unlocked lots of things, who know what will pop up? And there is no guarantee you will remember all of the key parts of the boss fights in whatever you get. Unless you play the game regularly and do group content frequently, there will be things that turn up where you kind of blank. And while it happens less often to those who are constantly running group content day in and day out, they also sometimes go... "???"... on some fights, too. It happens. To the most experienced players and to the least. It isn't a personal failing of great shame.
Those with more practice and a quicker memory for certain game design mechanics will do better at avoiding those "???" moments than those with less practice or who find the design philosophy unintuitive. And if you also are prone to getting anxious or upset when you make a mistake or feel like you've failed, this can really make running group content a dreaded chore rather than a fun experience. After you've been in the game a while longer, you will be able to see what comes easier for you and what is a struggle. There is always advice, but no magic bullet. Some folks love the game, but avoid chunks of it because it's too frustrating. Others quit. And still others muddle through parts of it when necessary to get to other parts they enjoy. Keep trying and see what works for you (and what doesn't).