I get called "selfish" or a "brat" if I try to set any boundaries with her or hold ones I've already established.
Yeah - the whole "you're being selfish" is a quick, easy way to hurt and therefore control. They use this phrase because it's worked on you and others, and it's a way for them to alleviate their own discomfort. Her comments are a good indication of how much she's hurting. It's intended to hurt you but it's not personal, and the sooner you fully accept that, the sooner you'll find yourself getting less heated.
All of us have limits, no matter how many tools we learn or how much we accept. When you feel yourself getting heated, check in with your body - is your pulse rate up? Is your breathing faster? Are you holding tension in your muscles? time for a break. Find a way to step away or give yourself some space.
My learned coping mechanism has been to get angry as it shuts the conversation down much more quickly.
I struggle with this too. It's worse when I'm already stressed, tired, or hungry. SWOE and learning the tools shared here really helps. It's hard work, and sometimes it feels counterintuitive when put in practice until you get the hold of it, but it works and it's so worth it!