Some advice, please?

I don’t really know how to explain to my mum that I’m sad. Whenever I try to she just says to wait through it and I’ll get over it. I often feel so sad that I just don’t feel like doing anything, and when I do, my mum thinks that I can’t be bothered enough to help her. It’s not true, I really want to help her, but I never have the energy. What I help her with is translating; we live in England and we’re foreign. My parents don’t speak English well so I translate things for them. My mother sells online as a job so I translate the messages, I also call the bank and other places in their stead. Sometimes I have to lie about my age in those situations (I’m under the legal age limit for interpreting) or pretend to be my mother.

I don’t want to talk to other people about this; I hate talking to other people about things that make me sad. Mostly because I’m not allowed to talk about things that go on at home but I also dislike when people know things about me. I dislike being around crowds. At lunchtime in school I’d go to an isolated place to eat. If there wouldn’t be one I’d rather sit outside in the cold than be around people. I almost feel threatened when people ask ‘are you okay?’. The teachers started noticing that I’m behaving differently. I told them nothing when they asked me why.

So, how should I tell mum? I don’t want to tell her exactly what makes me sad, she has enough worries without that. I just want to explain to her that it’s not because I’m being lazy, but I don’t want her to be asking questions. It’s be best if no one knew. I can’t explain myself what is the reason why I’m sad. People in my class are getting louder and louder everyday! They’re so annoying, some of them literally scream before lessons. I feel like I keep acting unnaturally but I can’t help it. What should I do?