Dear Milford,
and welcome to the family! It must be extremely distressing to be on the receiving end of such words!
How long have the two of you been together? The answer to your question is: fairly often, once the relationship is out of the mirroring stage. Often life stresses spark dysregulations and the pwBPD will feel overwhelmed and push the partner away.
You know your partner has BPD?Is there a diagnosis? All these things play a role in deciding how seriously to take it. Do not be surprised if your partner wakes up the next day and carries on as if nothing has happened. It is very common. The trick is then for you to process your posttraumatic stress disorder from the dysregulations. It takes time yet can be done.
Do let us know how it is going! And take care.