Despite every instinct fighting within her to stop him, to grab hold of him and keep him there, Lily didn't move. She knew this was best. There were only two ways for this to go down: Either Lily suffered through a life with Rez and his kids, or Rez and his kids lived on without her. Because there was no compromise in it, no way that Rez would send the kids off somewhere else to another family. She knew he wouldn't even consider it.
So she had to let him go. Maybe he'd hate her for it, he'd be angry and hurt and never want to see her again. She knew that she was selfish, but she also knew what she could do and what she couldn't. And she couldn't live the life he wanted.