I believe 'mental illness' is a literalized metaphor for very real personal problems that I most certainly do believe in, I just don't find it helpful to conceptualize these problems as being 'medical' in kind. If you feel this is a helpful concept to you by all means keep believing in it. But the lives of many show that seeing our problems this way is optional, and many have thrived after jettisoning such a belief. Of course, many professionals with personal experience of state of mind labeled as 'mental illness' don't believe in this idea either, http://www.madinamerica.com/2012/03/i-dont-believe-in-mental-illness-do-you/

Don't fall into the trap of claiming people who have been through life's extremes, and rejected a particular interpretation you hold dear, don't understand exactly what it's like to struggle with the most catastrophic states of mind imaginable in all of the human experience. When I say I don't believe in 'mental illness', I speak as someone who has been locked up, forcibly drugged, name-called as 'psychotic', been in places mentally where I was making meaning of the world in the most bizarre and terrifying ways. I'm a veteran of the most 'serious' stuff psychiatry claims is an 'illness'. When I reject this idea that these problems are medical problems, I do so with years of careful reflection and study, and with personal relationships with some of the most brilliant critics of these ideas in the entire world. I'm friends and acquaintances with people who have been though the system on every continent except Antarctica. I've spent ten thousand hours of my life at least, critically appraising psychiatry's ideas and networking with like-minded souls all around the world, like minded people both from the lived experience perspective and critically minded professionals as well. But above all, I don't force my interpretations on anybody, which is something that cannot be said for the average psychiatrist.