I have never been diagnosed with mental illness. But, there are people in my family who have been. I do know that I am a very sensitive person. I have trouble getting intense movies out of my head and I have a very strong imagination.
Towards the end of my faith walk I could feel my intensity in regards to "the spiritual realm" getting out of control. The Christian faith very clearly teaches that we do not war against flesh and blood but spiritual powers of darkness in the heavens (Eph 6:12). I think, hopefully, most people with religious beliefs are able to keep this in check. But if you are a person who is prone to irrational thinking, Christianity gives pretty good rationalization for delusions.
Here is an example of a popular Christian debater talking about keeping the faith through doubt. Notice his emphasis on Satan and that he "hates" you and is trying to destroy you (1 Peter 5:8).
This can be terrifying. I know for me, trusting the Church and Christian history, I have had many a night fearing my soul. I have spent countless hours discerning God's will and if something was a temptation of the devil. What I am saying is that it can go from a healthy sense of evil and bad things in the world, to down right paranoia.
This is an interesting article on the connection between religion and psychosis.
This was another aspect that threw me out of the religious experience. Once my basic family relations were straining, my marriage was lacking in some basic hospitable actions, and I couldn't seem to embrace friendships in a normal way, I decided it probably wasn't so important what was happening in the spiritual world. I needed to focus on THIS world. But, who has time? Especially as a Catholic. There's daily Mass (for devout followers, you know this is actually a goal), daily prayers, confession, examination of conscience, service, community, faith study, evangelizing, and more. Also, there is a level of living a good life as a Catholic that strains the most well meaning person. All of these things are supposed to help make us better people, but it seems that more and more I see it as impotent. Or maybe they are more like a placebo effect?
I do think that it is important to consider the lives we are living and to live them well. Like Socrates asserted, "the unexamined life is not worth living." It is helpful to strive towards our own well being and that of others. But we must keep it within reason. I am beginning to believe that the "spiritual world" is just a big excuse for believing our fantasies and inflating our self-importance. At some point, we have got to allow spirituality to accept its responsibility for the unfortunate consequences it has on fragile people.