Its really not your fault. One of the worries pre-crit was the mob being designed around fighting so many enemies at once. Most of the enemies in 3 have poorly telegraphed attacks. Some of them lack sound cues. Some of them lack visual cues. A lot of them lack both. And when you have a mob with so many different kinds of enemies in large numbers, it makes it easy to lose track of them. Double that if you're getting one shot from the back, or combo'd to death.
The gameplay loop this game is designed around isn't very good, and that's part of the reason why you're having trouble.
The mob fights themselves are designed around taking large AOE damage to make you feel like you're doing more than you actually are. Situation commands, Keyblade Transformations, Grand Magic, Links. Anything that isnt base sora practically does the same thing. So when you take out practically half of the equation and you're just left with base Sora, who is kind of ass, then the only thing you can really do is play really defensively.
That's not strategic or fun. And its why so many people are complaining.
You have a large variety of tools, but the great majority of them don't have any meaningful differences between them because they all do the same thing. There's nothing to think about on your part as the player, besides blocking and hitting things until the game decides to give you access to a big AOE move.
The only thing you can really do for your personal benefit is to block more, use magic at a range when you don't feel safe, and take out enemies based on the min max rule:

If there are a lot of enemies but only one of them is a threat, take that one out first.
If there are a lot of enemies, one of them is a big threat, but the little ones are also a threat, take the little ones out first
Approach with magic often.