Congrats on a great result, whatever you may think now, even getting to the bubble is a good thing because it's just 1 spot from ITM. It may suck, but there's always next time. Anyways, with no hand history I can't really say what you did wrong, but it's obvious you made a few missteps after that initial double-up if you went down to 4BB. Try writing down some hands that you think could've went better while you're playing, and then review them after. I always keep small notes on anything and everything when I play live, which is what I think everyone should do. Also, with an early double up, try to loosen up and widen your range just a bit while it's still early.