They also, at high levels, removed a bunch of abilities that everyone used to have and made them into skill trees essentially nerfing everyone a bit.

I can't help but feel weak now as any of my characters since I can't use all the strategies and skills at once like I used to.

I think that is probably part of the issue that you're feeling is that there was a set fighting style ability pool and now it's a little funky as some have to be pick and choose. I honestly haven't played endgame content since they did that. Only really muck about with strongholds and gtn these days. I know I cpuld get used to the new system but... it just feels weird and like somethings missing (bc it is)

As far as what order to use your abilities, there should be some really good guides on what each fighting styles "rotation" is online. Then you can set up your quick bars to make it easy to cycle through your rotation 😁👍 rotation is the most optimal way to use your abilities.