These relationships are very difficult and the more traits of BPD your partner has, the harder it is.

You don’t want to lose yourself, and it’s good that you’re aware of that. Even therapists who treat people with BPD (pwBPD) will often seek therapy; that tells you how emotionally challenging it is to interact with someone with this disorder.
As you’ve experienced with your partner, pwBPD are
very reluctant to seek therapy, and if they do, many of them will quit before it has been effective. The reason is as your boyfriend has stated—they’re too emotionally distraught to encounter the shame and pain that is buried within.
That said, if they commit to doing DBT therapy, many can see profound improvement in their lives.
One of the most difficult things for partners to accept is that their loved one is who they are: the good, the bad, the ugly. We can learn to minimize some of the damage we experience from their unkind words and find ways to cool down conflict. The bottom line is that they have a personality disorder and are always going to be difficult people, no matter what.