I realize I left off the last line I had thought I'd included,
"I have not been diagnosed with mental illness, and I have not avoided diagnoses."
I was wondering why there were no responses. And maybe that's part of the reaction from some of the posters.
The question was a legit one, in that I wonder if some cessationists may have mental illness. If one rejects the operation of the work of the Spirit, the Spirit of power, love, and of a sound mind, that may not be good for mental soundness. I'd expect that to apply more to those who actually encounter the work of the Spirit and reject it than to those who merely hold to a theoretical position on something they do not have direct experience with.
I wouldn't say all mental illness comes form unbelief or somehow rejecting the Holy Spirit. I think some of it may be demonic in origin-- or rather than mental health professionals may diagnose those with demonic problems as mentally ill. But I would not say it all is necessarily demonic. In Matthew 4, Jesus healed a variety of ailments including those who had unclean spirits and those who were 'lunatick'. It's treated as two categories. (I am not completely sure if 'moon sickness' equates to mental illness, but that's how its translated in one version.)
As with physical illness, an illness can be the result of sin, or it could be there for reasons other than sin. Jesus said 'Go and sin no more lest a worse thing come upon thee' to one man, but of the man born blind, He said 'neither this man sinned nor his parents, that he should be born blind'.