Her first reaction to conflict is to raise her voice. My reaction to her raised voice is to shut down and close her out.
To me, when she starts switching topics and bringing up the past its like being on the computer and having more and more windows pop up. I keep trying to close one window at a time but more and more pop up. Very overwhelming and nothing ever gets accomplished.
My H is the one who flashes anger and raises his voice. I am the one who shuts down. It's a struggle, but I admire you for recognizing the dynamic. It's up to you to keep yourself open and not shut down, even when it's uncomfortable. It's OK to ask for a short break and then pick up the conversation later. Even 10 minutes could help you gain control and open up.
My suggestion on the pop-up windows is, at a time that all is calm, explain to her that you are not a multi-tasker and that you like to discuss one thing at a time. You know she is good at keeping track of many things simultaneously, but you get overwhelmed, could she please help with that? If she reacts favorably, maybe you could come up with a phrase that you could say that she'd recognize in the heat of the battle, a code word or phrase, if you will, to indicate that you are feeling overwhelmed so she can adjust.
All this assumes she's willing to work with you, of course. Do you think she is?
If she's not, then maybe just try to keep gently guiding the conversation back to the main topic. Don't interrupt, don't cut her off, but you don't have to defend yourself on every pop-up, either. Stick to the original subject if you can, and ignore the rest. Maybe say, politely, I'd love to talk about that later, but for now can we make a decision on X?
