I've always interpreted Marx's concepts of alienation as talking about mental health issues. More recently people have definitely began to link the material conditions of capitalism with mental health issues, although a specifically Marxist writing on the topic doesn't spring to mind right now. It's also clear to me, both from lived experience as well as various things I've read, that dealing with this alienation labour is almost entirely the same as dealing with common mental health issues such as depression/anxiety, loneliness, and the existential question of what the purpose of my life is.
capitalist alienation is a significant cause/worsener of mental health issues, and he certainly did write about that (although the Solution is Revolution(TM) so there's no individual solution there)
The theoretical basis of alienation is that a worker invariably loses the ability to determine life and destiny when deprived of the right to think (conceive) of themselves as the directo inr of their own actions; to determine the character of said actions; to define relationships with other people; and to own those items of value from goods and services, produced by their own labour. Although the worker is an autonomous, self-realized human being, as an economic entity this worker is directed to goals and diverted to activities that are dictated by the bourgeoisie—who own the means of production—in order to extract from the worker the maximum amount of surplus value in the course of business competition among industrialists. —Wikipedia
For the dumb: Back around the time of Marx, people had two solutions for mental health. Either go to church, or get over it. No reason to assume he though differently.
Well, I'm probably only good for manual labor. So let's make a facility where you can put everyone who isn't good for anything else and make them work. Since people like me won't come voluntarily you'll have to create an armed force that can take me there against my will. That's ok though, because it's for the good of all of the "good" people.