Why does my girlfriend always want to break up everytime shes mad?

i love my girlfriend to death, but im afraid to say anything wrong because she gets really mad and says i dont love her and that if i want to be like that i can be single, i hate it! i dont want her to threaten me everytime something happens, ive been away from her since november so its been extreamly hard, but on march 15th im moving in with her so we can finally be back together, but its hard and its actually starting to piss me off that shes allowed to be mad and say things to me but if i get mad and question wether or not she loves me because shes so quick to pull the breakup card, then she reallygets mad and it sucks, i know its just the distance thing and in like, 3 weeks we'll be together and everything will be ok, but i have some pent up anger that ive been holding back on and if she keeps pulling that card i might snap and tell her some things i really dont mean but i do at the time because of how mad i am, how do i deal with her always pulling the breakup card and how do i avoid letting my anger out on her?