I am a psychotherapist. Mental disorders are often curable. Our mind, our psyche, our brain develop and change in every waking moment, one small increment at a a time. A good indication for this are mental disorders themselves. Their emergence is proof of our mind's capability to change - for the worse, in this case, but change nonetheless.

So in theory it should always be possible to change the other way around, to get significantly better to the point where the disorder is no longer present. (If you define a episode of mental health and wellbeing after a depressive episode as "managing" a still present disorder, then sure, they are incurable, but that's because that's part of your definition to begin with. The symptoms of a mental disorder can definitely disappear.) A more difficult question would be if our surroundings and social realities allow for so much change to take place. And sometimes, unfortunately, this isn't possible, since our society can be a fucked up place and economic constrains have an unavoidable influence on our capability to shape our own path.

Still, in my personal experience working with hundreds of patiens in different therapeutic setting, most people can (and do) reclaim their mental health, given supportive surroundings and adequate treatment. From your pessimistic outlook at mental health I will cautiously assume that you don't have those widely available to you. In this case you'd be somewhat right: Under such circumstances the possibilities to cure mental disorders are limited. Another complicating factor might be mental disorders themselves though. The feeling of "this is never going to get better, I'll never be happy again" is one most people with depressive disorders know all to well. So if we ask the affected people directly we will often arrive at the conclusion that the disorders are in fact incurable. And that's a horrible feeling for sure. I find it important to remember though that what our thoughts tell us in those dark episodes isn't necessarily the truth. In this case I'd argue it isn't. I've seen too many examples of the opposite, luckily.