One day I feel like I might snap at my really negative cousin
What did she do for you to snap?
I'm annoyed by her. She badmouthed me to my girlfriend, she never called to let me know that she will arrive late and I'm tired of cleaning up after her messes.
I'm sorry that she is a bad cousin and is making you miserable, but you shouldn't fight fire with fire. That only makes things worse, and you need to be calm and civil with her and tell your problems instead. You don't need to start more drama with her.
Oh shut up, I'm going to snap at her and make her feel bad.
I get your pent up frustration, but I believe it's best to not let it burst out in such a negative way. You could really hurt her and her family, and could ruin your familial ties. How about taking a deep breath and distancing yourself from her?