How to tell my mom this?

I've been struggling with OCD for about a year. I am afraid to tell my mom. I self diagnosed myself. I looked up all my compulsions and symptoms, and I am absoultley sure it is OCD. I am eleven. I know I'm not supposed to be here, but I need help! I am afraid I'll tell her and she'l either blurt out laughing or just be like, "Yeah, right, sure you do." My grandma also has OCD. My mom complains about her compulsions all the time, how annoying they are. My compultions are getting worse and worse, but I am scared to tell! I am afraid she'll be annoyed by mine. I try to hide them, but it's really hard. And no one else knows but me. How do I tell mom I have OCD?! Please help!