I saw your other thread. You poor thing. This has really got you in knots!
Try to separate out the issues. The panic attack - did that happen in front of colleagues or at home? If the latter, no bearing on credibility. If the former, the shock of being in an accident, or nearly in one, could easily explain it.
Going back to the situation in general, you can't seem to get past the 'is it the right decision/ will I be any good?' thoughts to 'what is it about the new job that has my juices flowing.' You can only see downsides. Tricky boss, potentially. Not a place you're valued.
But - flip all that on its head. That tricky boss has chosen you. The new firm wants you. You impressed them, with your CV and interview answers and personality. Your current boss, who in your head supposedly knows and values you more, only dangled the temp promo once you said you were off.
I say go. It's hard to leave familiarity but you are going to a really exciting opportunity. You just need to see it. Imagine if new firm suddenly says, oh sorry, our budgets have been cut and we can't take you after all. Guarantee that by October 1 you'd be plodding into the office thinking, Gawd is this really it for me?