There are two truths that friends have spoken to me, that have helped me tremendously. The first, a friend who went through something similar a few years ago. She told me that it helped her tremendously when she took responsibility for her choices. She choose her partner, she choose to love him, she choose to have children with him, and because her children were half him, she still loved him through her children, otherwise she would hate half her children, and that was not something she was willing to do.
Another friend, used the Transitive Property of Math to help me alter my perception. If A=B, and B=C, and C=D, than A=D. So, if I love my children (A), and they are half-Paul, and love their father (B), I can still love Paul (C), and if I love Paul, I can accept his pursuit of happiness (D). My friend theorized that there was a possibility of something beautiful coming out of this, if I am able to tweak my perception. Divorce doesn't have to be ugly, it doesn't have to irreparably damage everyone it encounters, and as long as I can focus on the love of my children, I can be happy for Paul. He is their father, he is a great father, and for the sake of his children, I want him to be happy.