So many parents, single and married/in relationships feel or felt like this though. That's what none or a vast majority of them will never admit. People really don't understand how hard parenting is. Some go into it thinking or predicting it'll be one way. In reality nothing prepares you for it.
While I think she's courageous for saying it, she also needs to go to counseling. Keeping those feelings bottled up is very bad, even deadly for the kids and the parents. Kids can feel when they're a burden. She doesn't want to put that on them because this is her issue not theirs.