Sage Stallone, 36, was found dead at his home in the Hollywood Hills in July (12), and a Los Angeles County Coroner subsequently determined the actor/producer suffered a heart attack.
The tragedy hit The Expendables star hard, and Stallone admits he is still consumed by grief four months after his son's passing.
He tells Britain's Hello! magazine, "I fall to pieces from time to time. You just do. It's a cycle you just have to deal with, like a virus that keeps coming back.
"You go through this time of thinking, 'It's really horrible', then you come out at the other end and realise there's not much you can do about it and you forgive yourself... Then you go through another cycle where you don't forgive yourself, then you get over it again... It's just an ongoing thing. And I don't think it ever goes away."