How to tell someone you have Social Anxiety Disorder?

I have social anxiety disorder, but no matter what I say to my Mother she just says that i’m ’ a bit shy, that’s all’
It is not uncommon for me and Mum to argue about things like going to the shops (Which I cannot bring myself to do) or catching the bus (Something else I can’t do) and I keeo telling her that I can’t do those things because I am terrified of them and I am more than just ‘a bit shy’. She either doesn’t belive me, which is stupid because I can’t even do everyday things, I can barely walk from my house to the car without being terrified. Or she just doesnt care, how can I make it clear to her that I have Social anxiety disorder and would benefit from her support?