I know this is an odd question... and to be honest, I don't really know whether anyone can answer it.
I have a mental health problem and feel that Christianity just doesn't explain it or support it.
What religion would you say does support MI best?
As a Gnostic / New Age Christian, I'd say Christianity. I think (not sure) I can understand why you would say Christianity, in your experience, doesn't explain or support it.
Though I'd amend what I just said by noting that Christianity isn't inherently best at explaining/supporting it. I think any of them can/do, but like all human endeavors (includes sciences), it may not consistently work out at level of ideal. Then again it may.
I recall when I was agnostic and had bouts with mental health issues, that any human that took time to listen, earnestly understand and offer help/assistance based on their life experience with or knowledge of community resources and psychological/spiritual understandings - that it helped me. Sometimes that may be a pastor, sometimes a psychiatrist, sometimes a counselor, sometimes a good friend.