What you might not realize is that EVERYONE messes up - in your foxtrot heat, perhaps others made just as many mistakes as you did, but didn't recover as well; or the judges were looking at someone else when you made your mistakes. Mistakes aren't necessarily deal-breakers in a competition.
As to upper-level bronze students competing in Newcomer - well, there are sandbaggers out there, at every level. Sometimes rules are liberal enough to allow it. My personal view is that, even if the rules allow it, I see sandbagging as not in keeping with the spirit of good sportsmanship. And, I've never had an instructor who encouraged sandbagging. But there are others who see things differently.
Take a deep breath, acknowledge that, for better or worse, this is all part of competition - and keep dancing!