Usher's marriage to Tameka Foster may have ended in divorce, but the singer doesn't consider the relationship a failure."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 everythingfrom 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."