Without knowing the specifics (and you are correct to withhold them from here) I'm not sure how anyone could possibly help you understand why the loan was rejected. Have you asked BBVA what the reason for the rejection is?
As I've made clear many, many times, I think that anyone who starts on this career path is making a huge mistake, and I wouldn't recommend it to anyone.
Define "huge mistake"? I've spoken to airline pilots well into their fifties who still love it and are still passionate about it. For them it certainly wasn't a "huge mistake". So, while it's true to suggest that for some people it will be a huge mistake, it's simply incorrect to categorically state that this is the case for everyone.