I don't know what to do.

I have been living with and taking care of my aunt for the past four years. She is a widow who lost her husband four years ago. For the most part, the two of us get along fine. But there are many days (like today) where I want to walk out the door and not come back. Whenever my aunt is pissed off, or angry at someone/something, I am the one she tends to (verbally) take it out on. She gets upset when she explains things and you don't understand her the first time she explains it. Which is exactly what happened today. She got upset with me because she was trying to get me to do something, and I wasn't getting what she wanted me to do. She yelled and screamed at me "how can you be so dumb". I blew up and yelled back at her to never say that again, and if she does I will leave. Things like this happen not all the time, but occasionally. And when they do I really want to leave. She said she was sorry, but I know she doesn't mean it, as this is not the first nor the last time she has or will do this. I'm stressed out, I'm angry, I'm frustrated, and I really don't know what to do. Talking to her is pointless as she is a narcissist who doesn't think she ever does or says anything wrong. When you try to call out her bad behavior, she cries and plays the victim. She has been going through a lot lately. She's been getting the run around from doctors, the anniversary of my uncles passing is coming up ect. I know that these things can affect the way a person behaves and can cause a person to lash out. But i'm getting tired of being the punching bag she takes it out on. My mom, sister, and aunt can't stand each other. And my dad is not physically capable of helping. So the burden of her care is pretty much left to me. She cannot afford, nor will she want to have someone else take care of her. I've been dealing with so much myself the past 4 years(stress, anxiety, depression). I have not been able to properly deal with any of it. I am reaching my wits end and I really don't what to do.