APD Aspiring Consultant - Advice

I'm a BME PhD wrapping up my studies and looking to finish up soon (in the spring). I've had a few consulting interviews, but with limited success. To give you an idea of my situation:

McK - Cut at Round 2 (naively, this was my first ever interview)
LEK - Cut after first phone Interview (next round would have been final interview)
ClearView - Cut after first phone Interview (next round would have been final interview)
Others: ICON jobs (cut at screening stage, may have been salary thing)

I also have a few others I'm waiting to hear back on.

I'm trying to "save" other interviews until I gain more experience. After each interview, I feel like I did slightly better, but still not sure if I'm "driving" the case enough. Sometimes I feel like my mind just goes blank, and I get nervous when I have to do the calculations. I know I'm capable of them, but something about doing the math in front of others just makes me incredibly nervous.

I guess what I'm asking is, do you have advice for how to "drive" the case. Like, are there any internal thought processes you have or tricks? I know I need to do this more, but when I get into an actual interview (not one with a friend), I feel like I'm not doing a great job of this. I suppose I might just need more practice on the math thing.

But just generally, I'm wondering how unique/ or common my experience is. For obvious reasons, most people don't want to share their letdowns or successes publicly, and I don't want to pester acquaintances that I know are going through the same process.

Reflecting on my experiences so far, I feel like I've done an OK job of getting interviews, and my resume has been good enough to get call backs. But when I actually talk to people in an interview situation, I disappoint them.

If anyone would have any advice to share with me, that would be great! Thanks in advance!