I attended a talk not long ago about grief that comes after being diagnosed with a chronic disease - now this mainly focused on a physical chronical disease. But I think it doesn't really matter if it's physical or mental illness. Grief is grief.
It's okay to grieve, grieving is a process we all have to go through at some point in our lives. And grief is different from depression and sadness - I think the best thing to do with grief is to allow it to be there, allow yourself to grieve over your loss. Be it loss of a childhood, loss of a future you envisioned, loss of identity or loss of a loved one.