Final Round Failure- Advice on succeeding in quantitative cases and powering through

Hey folks, first post so be chill with the excrement slinging if possible.

I recently applied, very late in the process, to Mck for their summer internship scheme. It was a total impulse, but I crushed the PST and first interview and felt like I was on top of the world getting that far without any real prep. Then it came to the final round, and I sat in front of the partner of the office I applied and I shat all over it. I walked out of that interview feeling like I'd embarrassed myself by wasting everyone's time.

I got nervous of the seniority, then screwed up the basic multiplication, didn't intuit the meaning behind the numbers and generally failed myself harder than if I'd introduced myself by saying "I only applied here because GS rejected me". To be specific, because I lost my calm, I wasn't able to use the numbers to progress the case, and then my mental math fell through. Yet I still really enjoyed meeting the team, the structuring/ideational aspects were fascinating, so I'm going to power on.

So firstly- does anyone have any advice on how to improve their mindset, especially when handling numbers, I'd really appreciate guidance on this
Secondly- I am sure it's been asked to death but what are people's favourite youtubers for mental maths? I am sure there are shortcuts I've missed and I am determined to learn more.
Thirdly - does anyone know if it is perceived badly to apply again for full time analyst positions after missing the internship? I am not hung up over it, I had my pity session and moved on, but I think I owe them a better interview than the one I gave. If not, I'll apply elsewhere as this is definitely the industry for me, but it would be nice to know. (for context, I am applying for analyst positions in a different office this time if that makes a difference)

Hope these are original questions here, good luck to everyone applying this season.

^^