...maybe I am indeed not up to the job...
Try not to think that way and you may save yourself some grief. It is really hard from this side to see somebody who has just scraped his rrrrs through every stage of training, but after a year or two of training doesn't get a rating and has wasted every bit of effort. I've seen this happen to motivated and seemingly intelligent people, and I've seen boxheads get ratings. It is not that you aren't "good enough", but maybe you are a round peg & the job needs a square.
Two experiences I had training people on a busy procedural sector destroyed my faith in my training ability for quite a while. One just used to freeze up when the pressure went on, exuding the smell of fresh !!!! when he was melting down. He could answer any quiz question, he could tell you 4 different ways of doing something AFTER he'd fouled it up, he could do any task if there was no pressure, but load him up & I'd have to edge away again from the "smell of death" & he would freeze at every decision.
The other just couldn't get a reliable scan together and never got out of the simulator. He was flash & smooth & appeared competent, but every sim run he would weld something or miss coord.
In both cases I'd identified the guy's "roadblock", but was unable to coach them through it. Maybe it was just not in their nature to overcome the problem.
Hope you find your niche.