Try explaining to her that you want to be able to talk to her (I'm assuming you do?), but a therapist can help you, they're trained for it. Your mum can give you support and love, but not necessarily the help you need. Maybe ask her to read up on depression, or print off some information to give her? My mum was reluctant to get me a therapist because she thought the same thing, but for me it was hard to talk to my mum because there are family problems and a lot of things she's having to go through, I just didn't want to make it any harder for her.
I'm unsure how it works in the US, but over here people under 18 can go to the docs and they will get you in touch with a councillor/therapist... is that not possible for you?

Molly