If there was someone I was interviewing who was terrific on paper but maybe gave a less than stellar interview due to nerves (but content-wise, was still on point), that is understandable. The interview is just one piece, but I need to know that person is showing up on day one with the requisite skills to hit the ground running. As my colleague pointed out earlier, some sites are just looking for a track record of very specific skills (it could be assessment, it could be research, it could be program development, it could be running groups, it could be working with college age populations, etc.). Try to convey your match (they call it The Match for a reason) as much as possible when you are answering every question. I remember during one interview, the interviewer had literally just pulled up my CV on his laptop for the first time and was reading my experiences back to me. I responded each time by saying something along the lines of, "Yes, that was a very important training experience to me...and I think it aligns with (or has prepared me for) your internship program because it helped me finesse this specific skill or gain this specific experience that is included within your program's rotation." The key is to be strategic with the very little time you have. Show them you did your homework on the site and that this is the perfect place for you. Jitteriness and nervousness can be overlooked in that type of situation.
As far as confidence, in this case I will say that practice (outside of doing more interviews) is important for you. I found it remarkably helpful as an applicant years ago to practice with current interns who still had the questions they were asked fresh in their memories. We did mock interviews and, not only did I learn what not to say, we developed easy frameworks to help structure my responses. I think the given advantage of doing interviews virtually is you can have a list of points pulled up on your screen to help jog your memory and prevent you from babbling. At the very least, be ready to give succinct, yet thoughtful responses to the basic questions (e.g., why this site, case example, theoretical orientation, multicultural awareness, what experiences you have with said population, discuss a challenge you have you experienced during supervision and how you resolved it, etc.) so that if they throw you a curveball it's okay to mess up on that one question. Go through your CV and ask yourself what skills and experiences you are most proud of because talking about these things during your interview will make you shine (you won't need to rely on a framework or list of points). Lastly, enter that interview knowing that you are the expert on your life and your talents. No one knows you and your professional experiences better than you. You just need to convince us how wonderful you are.