How do I tell anyone

I've had social anxiety for 3 years but I don't want anyone to know in case they treat me differently or think i'm a burden, so I lie a lot and fake smiles and laughing all the time because I don't want to bring someone down. I don't go to some things I get invited too because i'm too scared and my friends get mad at me for it. I feel like I can't tell my mum because she has depression and I don't want her to worry about me. Does anyone know how to tell someone about this, because I worry they won't stick around because i'm too much to take. Even if i've been there for them. I have a friend who, when we went to the same school, got angry really easily and would kick me or swear at me and runs away from school and the police would have to come find her. I stayed her friend through all of that and she's much better now but I'm too scared to tell her about the anxiety because i've always had to look after her and try to be a really easy friend to have, so she doesn't know how to look after someone else.