Welcome! Glad you're here, and glad your partner is engaged with therapy. DBT can make a huge difference.
There is a lot you can do, to keep yourself OK and to support your partner with appropriate boundaries. The BPD/codependent partner pair is super common. A lot of what happens in that interaction (which my wife and I share, and I'm working on), is behavioral patterns that the person with BPD wants, and thinks will make things better-- but actually makes things worse. Shouting back isn't helpful, but neither is passively accepting abuse (verbal or otherwise), caretaking the pwBPD, or protecting them from the consequences of their own choices.
Read through the lessons, if you haven't already. Keeping everybody safe is the first step. If your partner is self-harming, or talking about suicide, hospitalization may be necessary. Talk with her therapist about how to respond, and when you should think about bringing in emergency professional help. You'd need your partner's permission to talk to her therapist about it in the hypothetical, but you can probably call her therapist on the phone in an emergency, especially if you put it on speaker so the therapist is talking to both of you. If you don't have a therapist of your own, getting one would be a good idea-- both to understand your partner, and to work on some of your own stuff.
Official patient DBT materials, including the
skills workbook, are available on amazon-- my wife got the skills workbook before she was takign the course, and found it very helpful (formal DBT was *much* better, though). Emotion regulation and self-soothing skills, like the cold shower, are included in the skills workbook-- pages and pages of lists of suggestions. DBT-based books for partners include
The High Conflict Couple and
Loving someone with BPD.
Calming the Emotional Storm is a whole book on DBT techniques for emotion regulation, which was helpful both for my wife, and for myself. There are also many sites online (including this one and BPDCentral) that offer DBT-based self-help for people with BPD and their families.
I also agree wholeheartedly with Meili about boundaries and limit setting. That can result in a worsening of behavior at first, which is actually a normal response to someone *not* getting the response from others that they're used to (Think about trying to lift a box you could lift before-- if you can't, your first instinct is to try harder. Same with behavior.) But they're critically important in the long run, and not skills that many of us get from our families of origin.
I wish you and your partner the best of luck. You're on the right track.