Do you think she might benefit from talking to people going through the same things? I know that helps a lot of people, myself included. It might be an idea for her to try this website out. I think sometimes there's so much stigma around mental illness and those suffering don't really want to admit that they're ill, but once you begin to see that it's so common to suffer with mental illness, that it's not something wrong with you and that it's OK to be ill and need help... I think sometimes that can help somebody to face up to the fact that they have an illness and that there's absolutely nothing wrong in admitting it and asking for help.

Other than that, my only suggestion I suppose is to KEEP talking about it with her, let her know that you don't find it weird or wrong, that you want her to feel better and that you want her to be around for a long time because you love and need her, and that with help it would improve her life. She doesn't need to be in this black hole constantly, there is a way out, there is a light and you only have to reach for it.
If she begins to open up to you about her feelings perhaps she will then feel more open about going to a counselor or therapist in the future.

Do you spend any time with your mum socially? What is her social life like? Does she have friends, hobbies? What's her daily life like? Maybe she could make some changes so she is experiencing things she enjoys, bringing her maybe a little happiness? I know a lot of women here have tried things like going for a walk and gardening, and although they are small things they make a big difference. For me I enjoy going for walks, to get outside in the fresh air and the exercise creates those 'endorphins' which boost your mood, you really do feel better when you exercise.