Talking about it is key. I know you say she doesn't want to, and you don't have any control over that, but honesty and openness is the key to understanding and breaking the cycle. Once she can understand what leads to these switches to a difficult alter, she can begin to be able to anticipate them, and eventually learn ways- even in that difficult state- to change her behavior and cope with the experiences and feelings are differently.
The idea isn't to banish the alter, but to decipher to code of the alter's difficult behaviors and begin to transform those behaviors to healthier ones. What do these behaviors mean? What even more effective behaviors could be learned? Most all of this is her work. But you can help her. Identifying the triggers that she responds to by switching into this difficult state can go a long way toward positive change.
I'm thinking she is in therapy. If not, this would be very important for her (and you). At some point she needs to begin to realize that experiencing what you do at those times is a difficult experience for you. Sometimes a dissociative person can feel such pure, isolated terror that it can be difficult to realize that someone didn't intend to hurt, or that she may be frightening someone in her defensiveness; and when a dissociative person experiences pure rage (often misdirected), sometimes by the very nature of dissociation it can be difficult for that person to realize the full impact of their behaviors.
But becoming aware is crucial. Being in an adult relationship requires participation, negotiation, and compassion. In refusing to talk, she may be protecting herself because she is embarrassed or afraid, and creating a sense of safety and reassurance for her may help.
I suppose I would suggest that you consider a couple of options. One is to post a big huge colorful note on the refrigerator that prohibits, for everyone in the house, scary behaviors. The note could list alternatives. When I am angry, I will: Ask for a time out. Go for a walk. Leave the room to cool down, and return when I'm calmer. When I'm scared, I will: Ask for a time out. Take turns talking and listening. You get the idea. Another idea... Make clear your limitations, your needs, and the consequences: "I care about you so much. When you scream and yell, I feel scared and sad. The next time you scream and yell, I will need to leave for awhile until you can talk to me more calmly. Then I will come back. I want to hear what you have to say, but I need to feel safe too."
Sometimes, the idea that this is something for the two of you to work through together, can help everyone feel a little safer and happier. I don't think you can cope with this alone; if you have to cope with this alone, without negotiation and mutual change, it is unlikely that your relationship will last long-term. This is too much, from the sound of your note, for you to be alone with.Ask her for her help with the relationship, for her participation.
I wish you well.