That's not true. When a rational person notes he has a problem, they go to the docs to get help. Fed law recognizes that, so they don't disqualify folks when they voluntarily seek help. Only the ones that are involuntarily committed by a court, after having demonstrated they are a danger to self, or others are.
"How can a person be a member of a ‘well regulated militia’ if they are all hopped-up on anti-depressants?"
Other than the above consideration, you're right. The mall/school shooters certainly would not have performed well in any militia and I doubt many militia members themselves would've been comfortable having them around with anything sharp.
Militia service is not a pre-condition for having your Rights protected.
http://www.independent.org/newsroom/article.asp?id=1430
http://www.capmag.com/article.asp?ID=3295