Are there really that many Europeans suffering from mental illness? Or have the definitions of "mental illness" been lowered to include common, normal (but maybe not 100% desirable) mental states?
To paraphrase Ayn Rand: There's no way to rule sane men. The only power any government has is the power to crack down on the insane. Well, when there aren't enough crazies, one makes them. One declares so many things to be a mental illness that it becomes impossible for men to live without being determined insane ... create a nation of insane people and you can eliminate their rights for their own good.
Make your nation (or continent) into a prison or an insane asylum ... doesn't matter you still get absolute control (and if you declare your people to be insane you get to feel good about your tyranny).