A Port Jervis man decided to kill himself Monday in a particularly horrific way. A man managed to behead himself in a spectacularly gruesome suicide in the Bronx Monday. Tomas Rivera, 51, of  Port Jervis, tied a chain from his head to a pole, then hit the gas on his 2005 Honda CRV in Hunts Point, cops said.  He gunned it at 9:35 a.m., slamming into a parked vehicle on Longfellow Ave. near Randall Ave., police said. His body was ejected when he hit another parked car. He was decapitated and died at the scene.