posted January 29, 2012 11:45 AM            

A down-on-his-luck soap-opera actor took his own life this week after he was forced to put his beloved dog to sleep under pressure from his Upper West Side condo and became wracked by grief, pals said.

Nick Santino euthanized his dog, Rocco, Tuesday — on Santino’s 47th birthday. That night, his guilt over the gut-wrenching decision became too much to bear.

“Today I betrayed my best friend and put down my best friend,” a despondent Santino wrote in a suicide note, said close friend Stuart Sarnoff.

“Rocco trusted me and I failed him. He didn’t deserve this.”

The Brooklyn-born Santino — a struggling actor whose TV credits include “All My Children” and “Guiding Light” — adopted Rocco from a shelter several years ago.

The man, raised in an orphanage and foster homes, soon began to write about his pet on Facebook, writing, “I did not rescue Rocco, Rocco rescued me.”

But in 2010, his building at 1 Lincoln Plaza announced strict new dog regulations, including a ban on pit bulls. The ban didn’t apply to pit bulls already in the building, but friends and neighbors said Santino began to be harassed.

“People were complaining about his dog,” said neighbor Kevan Cleary, 63, an adjunct professor at Brooklyn Law School. “It was open season on him.”

Rocco couldn’t ride in the main elevators and wasn’t allowed to be left in the apartment alone for more than nine hours.

Santino was then threatened with a $250 fine for having a barking dog, neighbors said.

“The dog was not a barker, but somebody complained that the dog would bark,” Cleary said.

“He felt like he was in this battle because he was the only guy in the building with a pit-bull mix,” Cleary added.

Another neighbor, Lia Pettigrew, who runs a pet-care company, said, “Everybody knows that he had been harassed by the building management.”

The building’s management refused to comment yesterday.

After months of increasing anguish, Santino had the healthy dog put to sleep Tuesday.

Neighbors said a tearful Santino brought dog treats to the building’s doorman and said: “Give these to the other dogs. Rocco is no more.”

Dog owner James Steven Grant said Santino left two rawhide bones on his doorstep and earlier was seen tearfully giving away Rocco’s fluffy bed.

“Rocco was the sweetest dog in the world. Rocco wouldn’t hurt a fly,” Grant said.

A veterinarian told Santino that Rocco was becoming aggressive — and Santino blamed it on his own depression.

He spent Tuesday in agony over what he had done to Rocco.

“He was distraught and remorseful about putting down his best friend,” Cleary said.

The last phone call he made was to a former girlfriend at 2 a.m. Wednesday. Later that day, police found his body in his bedroom. He had overdosed on pills.

Rocco has been cremated, and friends said Santino’s remains will be, too, and they will be reunited.

“One way or another, their ashes will be together forever,” Sarnoff said.

Additional reporting by Aaron Feis, Jamie Schram and Andy Soltis

jsaul@nypost.com