How can i hold myself back from slapping her?

My mom takes all her frustrations out on me. I'm gonna skip the whole history of our relationship because its long and complicated but anyways just know that she over reacts at everything and goes rambo within a snap. Just a few moments ago I couldnt find an important document and I thought she had it somewhere so when I asked her she claimed she had no idea, but i specifically recall her holding onto it for me, so I kept on asking her if she had any idea where it was because it was urgent and i kept on asking her to at least check for me and instead of checking she all of a sudden flipped out and smacked me really hard and started screaming at me calling me a b*ch then I said "im sorry i just really need to know where it is!" so she slapped me again. I have a bit of a quick temper (guess i got it from her) and i need to resist the urge of going back in there and hitting her back. She makes me so livid. How can I control myself?