Kim Porter, who for more than a decade was in an on-again, off-again relationship with Sean "Diddy" Combs, is coming clean about the dissolution of their romance, saying his philandering was just one the reasons for their breakup in July.

"As Black women we sacrifice," Porter, 36, tells Essence magazine for its October issue, which hits newsstands Wednesday. (Part one of her interview was posted on Essence.com last week.) "We stand by our man, through thick and thin, through whatever. But if you feel like, 'Maybe I am getting the short end of the stick,' that's when a change has to take place."

Porter's rudest awakening in all her years with Combs, 37, took place late last year, when she was pregnant with the couple's twin daughters D'Lila Star and Jessie James, who were born Dec. 21. That was when a girlfriend informed her that Combs had recently fathered a baby with another woman.

"I wasn't even coming from the place of who, what, why," Porter tells the magazine. "I laid low and did what women do: I did my background work, collected information [about the other woman]. But I wasn't even on it like that because I was pregnant, and what was bubbling inside me was a blessing from God. I couldn't let my energy be on that. I didn't have time for any drama, trauma and bull----."

As Porter was to learn, the other baby was born five months before Porter and Combs's twins arrived. The mother was Sarah Chapman, 33, who lives in Atlanta, worked in fashion and knew Combs for several years, her attorney tells Essence.

Of Combs's behavior, says Porter, "It's not an original script. He's not the first man who's cheated. He's not the first man who's had a baby outside of his relationship. He's not the originator of this. But at this point in my life – I have girls now – it's a different program."

Porter expresses regret that she didn't first hear about the other child directly from Combs (who declined to speak to Essence for the article and did not return calls from PEOPLE for comment). "I would have preferred to find out from him because that's a man," she says. "I know it's hard for anyone to say to their significant other, 'I've gotten into some s--- and I got a baby on the way.' But men do get caught up in things; I'm not naive to that. Still, there's a right and a wrong way to handle it. Because, most of all, we were friends. Even if I couldn't have understood it as a woman, I would have understood it as a friend."

Ultimately, Combs did approach Porter with the news, but it was after their twins were born, she says.

"He told me that he may have gotten himself into a situation and he may have fathered another child outside the relationship and I said, 'Really? Well, I already knew. I'm glad you decided to be a man,' " recalls Porter. "I was like, 'Dude, this is so wack I can't even respect you right now.' And for me, once the respect is gone, I'm not even listening to you."

But as things finally played out, she says her choice to leave Diddy was simply because the relationship was not working for her. "I'm a Southern girl, and my home life is numero uno," she says. "I wasn't waking up feeling happy. There's no other way for me to articulate it, other than a feeling. I just wasn't feeling it, that's why I had to leave."