I know what you mean. They don't know. It's sad and sometimes it'll make you feel lonely, but truth is, you only know what depression is when you experience it. So your mother may be denying the truth to herself or may simply be blessed with the gift of not having an idea of how that kind of sadness feels.
Anyway, even if she was just heartless (which I find hard to believe but never say never) I can only suggest a few things from my own experience. If you're not underage and you can seek for help by yourself, just do it. Don't feel like you're lying or acting bad, you need help.
If you're underage, try to speak to someone else. If you think your mother would be against that, maybe it's better if you talk to like one of your teachers you trust the most or someone who doesn't belong to your family. Instead, if you believe you need help to tell her how you really feel, try to ask one of your relatives to tell her.. Like your cousin? I don't know.
I tried multiple roads before I understood that the best one was truly explaining what I felt.
Good luck.

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