And right there is the conundrum. You want people to seek treatment for mental disorders, but when such treatment is used against them, they won't seek treatment.
What? Someone would avoid seeking treatment for a mental disorder because he knows it might disqualify him from buying a firearm? Those are EXACTLY some of the people who shouldn't have access to guns.
2A says "right to bear arms shall not be infringed". Should we make an exception for those known to be mentally unstable... and have been prescribed psychotic drugs?