It's a real Catch 22 in the flying business. If you're not feeling well mentally and seek medical help, you're out of work until you can prove to the feds that your disability was transient. If some shrink pens a diagnosis, however speculative, your career may well be over.
Over the years I've know a few colleagues that have had mental issues. Some returned to flying, most did not.
If your poor mental state can somehow be blamed on drugs or alcohol, you may have some job protection in the U.S. under the Americans with Disabilities Act.
There was a similar inflight crew mental issue on QF months ago:
http://www.pprune.org/forums/showthread.php?t=251530