Leia says in both film and novelization
“That’s why I wanted him to train with Luke,” Leia said. “I just never should have sent him away. That’s when I lost him. When I lost you both.”
This quote is a bit confusing at first but I think I understand it. Leia initially wanted to send Ben to train with Luke because she sensed him having trouble with the Dark Side. But actually sending him away only exacerbated the problem due to reasons we don’t know.