Just my opinion..
You are not born with the skills to control, but the NATS selection process seems to choose people who are most likely to pick it up quickly and less likely to fall down when it all gets difficult.
I don't think there's a better test of whether you're fit for the job than going through the selection process yourself and seeing what happens. The fact you have the aptitude to fly and learn for the ATPL exams sets you in good stead.
Some people find the training easier than others, but I think that from your post you sound like the sort of guy who would put in 100% effort in order to get through everything and that's a far more attractive quality in my eyes than someone who's always aced every exam and never had a problem picking anything up. I think one of the important things about trainees is attitude to criticism and failure. If you can deal with criticism well, take and act on advice and be strong when things don't go too well you'll do well in the training programme (from my 4 months experience of it anyway)
The college is full of the most bizarre set of backgrounds and completely random levels of academic acheivement so don't think that school/uni performance is really too much of a factor which determines how well you will do.