I can relate to your pain cariadmon, I had the same thing happen to me at this point last year - stage 3 failure despite feeling like the whole thing went great. I couldn't put my finger on why it went wrong at all and HR were absolutely useless and didn't get back to me with my feedback despite 3 attempts at getting hold of it.
I've just tried to forget about ATC altogether over the last year and plug away at the job I no longer want to do! But the time to reapply has come around quite quickly and in that time I've come to realise that perhaps my teamwork could have been better and my technical interview could have had more prep. I just worked on all aspects of my life so now I feel more ready than ever to try again (I'm currently waiting for my personality test result). If it's what you really want to do then it's worth waiting for again.