Cracking under stress in an interview

Just came out of an interview. Interviewers were super kind. Had me walk through resume etc + some behavioral questions. All good. Then was asked a very basic multi-step problem of finding some key numbers for company (think EBITDA, EBITDA margin, FCF, etc -- basic stuff). Let me tell you, I have done a TON of preparing, and I knew exactly what they were asking for and how to find it. Problem is, when someone is reading out numbers in the heat of interview - I just completely lose focus. I'm pretty good at mental math normally, but I swear when I'm being interviewed my IQ drops like 50 points and the easiest mental math becomes completely impossible for me. I make extremely stupid errors, like forgetting components of equations or making calculation errors and stuff like that. And it just compounded on itself, as I made one mistake I kept making more.

I feel like I bombed, but it's so disappointing because I could have gotten the right answer 10/10 times if I just stopped panicking. The worst thing is, there isn't something tangible I could improve on for the next interview. Like, if I did not know a certain key term, I could go and study afterwards and make sure I knew it for next time - but no amount of mental math practice is going to help because I can do it normally. Jeez I just feel pretty disheartened.