Looking for 2 months off due to mental health, what to say and show?

hi everyone,

So all my life I have suffered with anxiety and undiagnosed ADHD which I know I have but just haven’t gotten my diagnosis yet, Currently in the process of getting it as I’ve seen a huge impact on my life. Unfortunately the past half a year since I’ve been working at my job, which I do indeed enjoy and it is very enjoyable, my mental health has taken serious hit that I don’t feel myself and internally unstable. Looking to go home for two months to simply get my head right.

Now I understand two months is a very lengthy absence especially for sick leave so how do I go about asking management/HR for that amount of time off. I have a list from the Internet of many things to prepare but what would anybody here suggest that I should have or collect that can prove I need a lengthy absence due to my own issues.

Thank you, I am still going to speak to my manager tomorrow whom I dearly trust but would appreciate any advic.