Practice and breathe. Realize that even if you're doing well, interviewers can still perceive you poorly, and the process is weighted to reflect how much more

Practice and breathe. Realize that even if you're doing well, interviewers can still perceive you poorly, and the process is weighted to reflect how much more painful a bad hire is than rejecting a good hire. Practice is going to improve your performance and put you at ease with the process, and give you a better understanding of when your performance was not up to snuff versus just not meshing with how the interviewers wanted to interact with you.