I'd suggest the odds are against everyone who applies for NATS at any time, only a limited number make it through, you have to work hard and really want to do it to get there. That said it also shouldnt be seen as the only option, if you fail first time around, use the 12 months wisely to improve yourself and make it your mission to prove to the recruiters that you are worthy of a job offer. Your application won't be viewed positively or negatively as regards other candidates, its what you have changed about you that is the pivotal factor.
And for my two pence worth surrounding personality tests... surely you just answer the questions?
I mean sure theres a lot of them, and some of them may have answers that dont necessarily apply to you, but on the whole theres an answer to suit the type of person you are. Sometimes I think people may suffer in these tests cause they get caught between giving their personal answer and the answer they believe they want NATS to hear... May be wrong on that and stand to be corrected, but for me thats a sure fire way of confusing yourself and showing up badly to the recruiters. Just be yourself...