There was a time when we were all considered mentally ill (still are technically but that is changing - it is not longer popular for someone to be diagnosed as mentally ill because of a lifestyle choice and we have the homosexuals to thank for fighting that war). The truth is that mental illness is a very vague and difficult thing to define adequetly.

On topic: I agree with the above. Communicate everything you can with your partner(s). That way they know they can deal with it. Bear in mind, in a long term relationship where one has a degenerative or relapsing/remitting condition there is a chance that they may have to sign the paperwork to have their lover committed or similar. This is a massive thing which many are scared of doing so the more you talk about it the better.

Most people, once informed of a condition like this, accept it and deal with it and learn to work thier life around it.

BTW, if it helps make you feel better, I am of the opinion that anyone who is classified as psychologically 'normal' is about as unusual and 'mentally ill' as anyone else. Its such a rare thing to be 'normal', everyone has some quirks of personality or little mental problems which distinguish them from everyone else. In a relationship, you have to get used to another person's quirks and you could see mental illnesses as merely slightly more serious quirks that happen to have an official diagnosis and the benefit of some form of proper treatment. You rarely get that for being simply a jerk