Does every condition need medical evidence or an official diagnosis?

Hi, I suffer from a mental health problem as a result of a pre-existing disability. The thing is, I've never been to my GP about it and wouldn't be very comfortable in bringing it up because I'm worried that they won't believe me and if I go now to get a diagnosis and evidence, they probably won't believe me.

I just think that university will open up a lot of doors for me and I want to make the best start possible so I'd like to get all the support options available to me.

Can I just mention my condition to the assessor or do you think they'll want to see medical evidence?