But that's kind of the crux of the matter, isn't it?
First off, we still know so little about mental health that we can't accurately tell which is which. And it strikes me as really harmful to categorize people's issues as 'real' or 'not real' based on vague notions. Autism, for example, was initially seriously considered a result of bad parenting, and the consequences are noticeable to this day.
Second, even if we can distinguish between, say, depression because 'raised with too much self-esteem' or 'chemical imbalance', this is mostly relevant for treatment, not diagnosis.
If the person in question cannot find their own way out of the situation, the problem is by definition 'real'. The only difference might be how the situation is handled. In fact, in many cases the treatment is the same, regardless of how someone got there.
I've seen a lot of harm caused by people arbitrarily distinguishing problems as either 'nature' or 'nurture' or 'internal' or 'external'. This distinction is not only extremely vague, it doesn't actually matter to anyone except those who are developing or implementing solutions. To everyone else, I'd advise compassion, support, and avoiding judgment.
That said, you do have a point. Personally I've become more and more convinced that for various reasons, some good, most bad, our current treatment models err on the side of adjusting yourself to fit your environment, while the most beneficial solutions I've seen involve shaping your environment to fit 'you', or at the very least a combination of both. Regardless of whether an issue is 'internal' or 'external' (again, I don't think making the distinction is as valuable as it seems).