Vince Neil Breaks Up with Longtime Girlfriend Over Alleged Affair

Vince Neil and his longtime partner, Rain Hannah, are officially done ... and, we're told he's the one who pulled the plug, because he thinks she was cheating.

Sources close to the Mötley Crüe front man tell TMZ ... he broke up with Rain, after nearly 15 years together, in response to one of his assistants claiming Rain had been having an affair.

We're told this alleged romance was something of an open secret among Vince's inner circle, and no one wanted to tell him -- but, the assistant recently got fed up, and decided Vince deserved to know.

Important to note ... we have not confirmed there actually was an affair, but we're told Vince is absolutely buying it.

Their split comes a few months after Rain survived Neil's private plane crashing in Arizona, along with her friend Ashley and several dogs.

Rain broke 5 ribs after VN's jet crashed into Vince Vaughn's plane -- which was parked near a hangar -- killing the pilot of Neil's plane.

Scottsdale Airport officials said their preliminary investigation appears to show the left main landing gear failed as the plane was touching down, causing the collision.

We later learned Vince skipped the flight because he and his friends were in Florida hanging out for an extended birthday celebration.

Rain was flying to Arizona to take part in a horse event, but Vince didn't have to be there until several days later.