Could Shelley Long suffer from bipolar disorder?
This could explain her apparent mood swings on the set of Cheers. And she did attempt suicide back in 2004, which would lend credence to the notion that she has serious emotional baggage/issues. Plus, a lot of undiagnosed (and diagnosed) bipolar people abuse prescription drugs. They are basically self medicating.
http://www.suicide.org/shelley-long-attempts-suicide.html
https://www.datalounge.com/thread/12398176-shelley-long