John Lennon's son felt "cast aside" when he embarked on a relationship with Yoko Ono.
Julian Lennon has recalled how he felt the late Beatles legend had "disappeared off the face of the planet" when he divorced his mother Cynthia in 1968 and got together with the artist, because after that they had very little contact.
Julian said: "Suddenly my dad literally disappeared off the face of the planet. At least, that's how it seemed to me.
"He and Yoko Ono were deeply and publicly in love. And I felt as if my mum and I had been cast aside."
Julian, now 56, felt grateful that his dad's bandmate Sir Paul McCartney didn't "forget" about him.
He added: "Not everyone forgot about us, though. Paul wrote Hey Jules after dropping in to check how my mum and I were doing. (Obviously, the title of the song changed to Hey Jude)."
The Saltwater singer didn't speak to his dad - who was shot dead in 1980 - for almost a decade but credits his "gentle" mother for helping them repair their relationship.
He told Observer magazine: "Maybe 10 years passed during which my dad and I barely spoke. I was very angry about how he left the family.
"It was thanks to my mum that we started having conversations again. She was such a gentle soul, never vindictive in any way, shape or form. She always wanted me to have a relationship with him."
Julian still strives for "forgiveness and understanding" towards his late father and holds his memory "dear", though he can't "forget" John's behaviour towards Cynthia.
He said: "I try to remember my dad as fondly as possible. I strive for forgiveness and understanding in that area of my life, for the difficult times he put my mum and me through. I loved her more than anything and can't forget how poorly he treated her.
"But our relationship was getting better before he died. He was in a happier place. He wanted to reconnect, not just with me but with the rest of his family. He never got a chance to do so. Even now, almost 40 years after he died, I hold my father's memory dear."