I went through the process over a month ago. The key thing to remember is that McKinsey is very different from most other companies in that the interviewers genuinely want you to succeed unless you've managed to create such a negative impression for some reason. Remember, they will be extremely nice and understanding.
To reduce performance anxiety, I recommend practicing more. RocketBlocks is a great resource for repetitive practice for case prompts.
Also, this is going to get downvoted heavily based on how much these programs are idolized in consulting forums, but Case in Point and LOMS actually screwed with my casing substantially. I had to spend a week to reset my knowledge after reading it because both programs had dropped my level considerably. I found that learning about basic industry trends, business knowledge, and formulas and combining that with common sense was the best solution for me. Maybe the programs will work better for you, but ultimately the case studies are a chance for the interviewers to see your mind in motion, not for you to regurgitate scripts from a program. Maybe sure you showcase that.
I don't know if you have a behavioral portion or not since I'm not sure what position you're applying for. However, if there is a behavioral portion, remember that McKinsey really just cares about your intrapersonal impact. Can you get along with others? Can you work well with others? Can you lead others? Do you have empathy? Flashy positions or accomplishments will not compensate for the lack of intrapersonal substance.
Feel free to reply here if you have any follow-up questions. Cheers!