People with BPD can do a real number on our self-esteem.
It sounds like she is projecting her own stuff onto you (feeling inadequate). Even if she's figuring things out as a new mom, she likely feels some sense of being out of control.
whenever I try to talk to her about it she gets defensive and argumentative
Do you feel comfortable giving us an example of what she says and how she says it, and how you respond?
Maybe we can help walk with you and find ways to make things from getting worse.
Are you getting any time with the baby?