Why do I suck so much at RDM?

[Galvuu]

That's because you're thinking too much.
Don't get me wrong, many jobs have decision making components and need it all planned out (I'm look at you, SMN and BLM).
But these meaningless things ("what spell do I cast next?") have to be ingrained.
I'll glance at my mana (and TP, if I sprinted) to determine what filler spell I need to cast when I enter my UI cycle (Foul or something else if I have the mana for it/know it'll be a fast tick). You need to look at stuff like your Aetherflow cd on SMN.
But I don't think "do I Fire next?"- I instinctively know this (and even if I have to Fire early due to mechanics).

Read that guide, or any guide, or read the tooltips and do some napkin math yourself to figure out what's optimal.
And then go bash a dummy for like an hour, or two hours, or how many hours you need (I'm not joking with this).
Eventually, you'll get bored and distracted and "zone out" of what you're doing. When this happens but when you regain awareness you notice you're still keeping up with the correct rotation, then you know it's ingrained.
All I keep track off on RDM are my ogcds and my mana just as I start the melee combo (to pick the correct finisher). Everything else is already second nature.
It's all practice, really.