I recently heard someone say this: 'If it's meant for you, it won't pass you by' - So if that firm is your future - there's nothing that you could've said or done that would've jeopardised that - Even if you didn't perform at your best, the firm still won't reject you because it was meant for you, so all their assessment criteria/marking criteria or the quality of the other candidates, lined up in YOUR favour, even if it wasn't your best performance.
I hope I'm making sense, but this has been a nice way of not overthinking for me! I hope it helps