Hi whiplash
Dealing with A BPD parent can be quite difficult indeed. Having a support network can be invaluable and that's why I'm glad you have the support of a therapist.
It's unfortunate though that your dad didn't stick with his therapy, perhaps he will return in the future. Time will tell. Do you feel that your dad has ever in any way truly acknowledged that there might be something wrong with his behavior, specifically the way he treats you?
Have you discussed this latest incident with your therapist?
There are certain tools that can really help in communications with someone with BPD, such as
S.E.T. and
D.E.A.R.M.A.N.. Are you familiar with these tools? They can help you express and assert yourself in a way that minimizes the likelihood of (further) conflict while maximizing the likelihood of getting through to the other person. These structured ways of communicating can also help you stay (more) calm yourself.
Take care and welcome to bpdfamily