I want to tell my mom i think i'm depressed but don't want to tell her because she might think she failed as a mom to make sure her child is happy or something around that. any advice on what to do?

You just did it well: Your words here sound like a good way to do it. If you start with how much you care about her not feeling bad - reassure her that you are not blaming her - it might be much easier to say that even so, you'd like some help with symptoms that really need professional help in addition to a mother's caring and concern. That doesn't sound to bad to me - and i'm a mother.