How can I pass interviews when I'm not a full blown normie?

I'm very book smart. I did a respected degree. But on Monday I had a graduate assessment centre for a prestigious government department and probably failed, probably due to bad social skills and / or nervousness.

If you gave me the interview questions on a piece of paper and told me to type up answers for tomorrow, I'd easily do perfectly. Many online application forms require written answers to competency style questions and I've passed them. I'm not a sperg who rambles for 10 minutes. I understand what they look for in competency questions. But when I'm in interviews my voice gets "clogged up" (a lot of saliva in my mouth, I'm quieter, maybe speaking quickly sometimes). I might stutter or hesitate in my thoughts while talking at one or two points in a half hour interview (not to any huge degree but obviously HR normies are ruthless in their culling of non-normies so I assume that it is instantly fatal for my chances when it happens). I wouldn't say that I come across as mentally ill or crazy, merely nervous at most. And almost every graduate recruitment process in the UK is made up of only competency questions (this is true for public sector all the way to very technical engineering positions).

I want to reiterate that I'm NOT a STEM autist who has never read a fiction book, who has trouble understanding facial expressions etc. I'm simply a guy with no friends or social life at all and who hasn't had either of those for over five years. I have failed over 20 graduate or intern schemes within the past 3 years, most of them with tip top tier companies and positions that even Harvard grads would love (and I went only to my local university you probably haven't heard of). Front office investment banking, etc. I should say that I have passed graduate phone interviews but never face to face ones (so I've never passed an entire process).

Is the graduate recruitment process really so ruthless in culling anyone who isn't a normie?