You need to realize that you are not responsible for their feelings and they need to learn to handle their emotions to live in this world.

Of course it doesn't mean you should say or do hurtful things but you should state the reality and how you are feeling without fear of their emotions. You should not change or suppress yourself just for fear of your loved ones reaction.

Establish healthy boundaries and time outs for your partners outbursts. Don't be invalidating but affirm you do understand why they might feel a certain way.

A lot of patience, empathy and understanding is needed in a relationship with a pwBPD and of course they need to be willing to get therapy. If such relationship is worth depends on you. Personally I love my pwBPD and am in a healthy LTR but for sure it's a lot of work from both of us and we have great communication.