Kelly Clarkson is opening up more about her “horrible” divorce from Brandon Blackstock, admitting co-parenting their daughter River, 6, and son Remington, 4, has been “tough.” Her confession comes on the heels of the news that Brandon, 44, is requesting $436,000 a month in child and spousal support (a total that tops more than $5 million a year) and that Brandon’s father, Narvel Blackstock, who owns Starstruck Management Group, has filed a claim against Kelly alleging she owes them $1.4 million in unpaid commissions. “The situation with the kids, the money...this divorce keeps getting nastier and nastier,” a source tells In Touch . But Kelly, 38, “is staying strong. No matter how ugly it gets, she’s vowed to not let this divorce destroy her.” Still, the there have been times…