Let’s take the example of depression. When people with depression get the proper anti-depressant it can seriously aid with the disorder. This would suggest that depression is something chemically based which can be addressed with medication. Our senses and knowledge of the universe as we experience it tell us then that depression is a physical/chemical illness which can be treated by physical pills and chemicals. However, what if there is actually an invisible imp pounding on people’s brains causing their depression, and the imps happen to be put to sleep by serotonin and other such drugs? Many will laugh, but the sad fact is there is no way to address this. All we can ever do is make claims based on statistics and the assumption that humans are seeing the whole picture.
I have depression and have found that you can change it overtime incrementally. [there is no on/off swtich for it; if I were to speculate, I'd say its because you have to form new neural connections in your brain and that places a physical limit on the speed at which the brain can change.] I'm obviously relying on my own inner "sense" of what is going on and by trial and error have changed it and recover.the inner experience is a sense of "pressure" in your head, and when its bad, you feel like your banging your head against a brick wall or that your brain is being tightly wrapped in some kind of cloth. at the moment, it feels like I'm in a large open space, with little stress or pressure. [the mood I'd compare it with is of being by the sea side on a warm day with the sound of waves rolling up a beach.]
Because of the problems you stated, I think this might actually be pretty rare. psychologists simply don't "know" much about mental problems as we're still a long way away from a scientific understanding of the mind. [what will be of interest is I assume that it is possible, but have no firm basis for doing so beyond a materialist assumption that all consciousness is a product of the brain. so if we study the (physical) brain, we can study the subjective experience of the mind too.] I can't make any absolute knowledge cliams, but just see "what works" and have arrived at the conclusion that I was "conditioned" to being depressed and am therefore having to learn new habits overtime to see how it goes. As long as it works, its good enough for me to do and to try. I'm never going to be 100% sure and I can live with that.
Often it is the absolute cliams to knowledge, to right and wrong that makes depression worse for me. learning to handle controversial ideas is useful because alot of the depressions stems from thinking other people's opinion of you were the same as physical barriers and are "real". or perhaps that something is "impossible" because everyone else says so. overtime, I feel more is possible and my mind "opens up" more and feels less stressed. it sounds crazy, but everything sounds crazy when your dealing with the brain. letting go helps. The puzzle for me, is that I am a determinist, but have no evidence for rejecting free will; its two interpretations of the same thing, but determinism is the more useful of the two (as it means you can look for specific causes of mental problems and change to see what happens).