Sorry to hear some people are getting bad news at the moment. You can get feedback to find out why them didn't think you're suitable - wait until the news has sunk in a bit and arrange a time to speak to HR.
binbrain: Unfortunately it's not just about mental capacity to cope at the college (and if I'm being truly honest, nearly all of us at the college probably didn't really know how hard it is sometimes until we were there doing it). There's a lot more that they're looking for - for example, it can be about how you interact with people, how you've dealt with situations in the past, the way you assess risks (some people are generally risk-takers, others risk-averse, and not something you can change easily), etc etc.
There's quite a lot of people at the college who got through to Stage 3 on their first application, failed, got some more experience or just had a better idea of what was going to happen (for some, it's the first time they've had an interview like that), re-applied and were successful 2nd or 3rd time round.
Don't know how old you are, but if you're under 25 and willing to work overseas, you can apply to Eurocontrol. Alternatively it seems it's hard to get into the non-NATS airfields without at least some ATC training - there are two colleges where you can pay to train, but it's expensive, and unless you can make arrangements before you start, no guarantee of a job once you finish.
Good luck with whatever you choose to do.