Former "Prison Break" star Lane Garrison is a changed man after serving two years behind bars -- claiming his lengthy prison stint was "a painful experience that I would wish on no one."

Garrison -- who was convicted of vehicular manslaughter back in 2007 for a drunk driving crash that killed a 17-year-old boy -- opened up in L.A. yesterday ... calling his experience in the clink his "worst nightmare."

Check out the clip ... and remember, kids -- never, ever drink and drive.