The 'Behind Closed Doors' hitmaker - who has two children, Junior, six, and Princess Tiaamii, four, with glamour model Katie Price - was shocked when he first saw his kids with his ex-partner's new boyfriend Alex, and woke up in the middle of the night with a "horrible" feeling he found difficult to deal with.
In an interview on 'Piers Morgan's Life Stories', he said: "It killed me, that night I woke up about two in the morning and I felt like I had been hit on the head with a hammer. I started to panic. It was horrible."
Peter also revealed he still loves his ex-wife Katie - who he divorced in 2009 - because she is the mother of his children and he continues to be "tortured" by the failure of the relationship.
Asked by Piers if he still loved his former partner, he said: "As the mother of my children, yes.
"I've moved on in leaps and bounds in one way but in another way, I'm still a bit tortured inside."
Peter - who also acts as a stepfather to Katie's other son, Harvey, nine - recently claimed he and the former glamour model were "trying to be friends" after a long period of strained relations.
He said: "My ex, Kate, said on [UK TV show] 'Loose Women' last week that I was the only partner she's ever had who's bought her presents. It's really nice that she said that about me. We did have some really good times together and we are talking at the moment and trying to be friends."