How to tell my mum im bi?

Im bi, yesterday night I told my best friend and she was perfactly fine with it, she said it doesnt change who I am and its not going to change anything. She acted the best way I could of hoped. I really want to tell my mum, i know that she would be perfectly fine with it and i know atleast 97% of my family would definatly be perfectly fine with it, the others probably would be but im not sure how thye would react. I know my mum would be fine because my cousin is a lesbian and when she came out my mum did not care at all, she kept saying to her its who you are that matters you cant help it, I dont know why im scared of telling her. I really don't know whyy at all but i am terified! please help!