Hi Tania,
Try not to shut your mother out of your life as if even as you say she brings you down with her own depression.
Try to limit your time with her, have you tried talking to her about the way you are feeling without bringing up her own depression?
As A Mother myself, I would hate it, absolutley hate it if I thought that my own depression would spark off theirs. I know it has an effect on them as they are concerned for me but I promise you that if i thought that either of my children were suffering in this way my 1st instinct would be to protect them.
Maybe if she knew how deep your depression lies and you told her that you cannot be around any negativity for a while so you can heal she may be able to hold herself together because she loves you.
i do hope so. Please talk to her.
Keep it about you and the way you feel and not accuse her as she obviously has her own issues to deal with.
Good luck. x