"I appreciate the relationship for what it was," he says. "Even though it didn't work out, I'm glad we got together. I love her as the mother of my children; we just couldn't be married."
So what went wrong? Usher says that shortly after the birth of their second child, he and Foster started to disagree over everything from parenting style to his busy schedule.
"We're two different people," he says, "and we have two different goals. It just was never going to work out 100 percent."
The singer, 31, who sat down with PEOPLE in his Atlanta-area home, says that the couple's priority continues to be their children. (He and Foster share custody of Usher V, who turns 3 in November, and Naviyd, who turns 2 a month later.)
"We had to put our issues aside and focus on raising them," he says. "There will never be another mother for my sons, and I'll always be their father."
Life Father, Like SonsRaising two boisterous boys can be challenging, especially when they take after their famous dad. "Usher [V] is always in everything," he says. "He's an instigator. He's the one who leads his brother astray. I was like that when I was his age."
Naviyd is more of a follower, doing whatever his big brother does. "So if one gets in trouble, the other one does, too," says Usher. "They both have a lot of energy. They can wear me out."
Fortunately, Usher has a babysitter nearby – his mom, Jonetta Patton, lives around the corner. "She spoils them," Usher says with a sigh. "She lets them do everything that she wouldn't let me do. Sometimes, when I'm scolding the kids, I sound just like my mom. I'm like, 'how did Jonetta get in here?' "
His CareerUsher, who released his new EP Versus last week, says that his work continues to evolve as he evolves in his own life.
"My music is about where I am at the time," he says. "In Raymond vs. Raymond, I was going through a lot of things, and it came out in my music. My marriage fell apart and I was suddenly a single father. But now, with Versus, I'm singing about being a dating father."
And will he sing about falling in love again? "I still believe in love," says Usher. "I believe in marriage. When I fall in love again, everyone will know. I'll sing about it."
For more photos and details on Usher's life as a dad, pick up this week's PEOPLE, on newsstands now