Nothing is right anymore and i feel helpless to the one i love. Can somebody help?

Im a 15 year old boy that needs some advice. I don't want to sound like a loser but i haven't lived the best life. Growing up i was abused by my father, it doesn't happen much now just because i think he knows ive grown up. Im big for my age. Im 6'3 and 230 pounds. Ive been thrown threw walls, ive had eyes swollen shut, ive pissed blood, and ive woken up in hospitals. I watched my parents trying to kill each other before. I believe the craziest one is when they chased each other around the yard with there cars. BUT IM NOT HERE FOR ME. Its my girlfriend. we've been dating about 5 months now. Shes the world to me. Were very happy with each other. But for about the past week shes been acting strangely. She has had a life kinda like mine. She tells me anything but this time she did not tell me til later. I told her id leave her alone about it and ill let her tell me when shes ready. Then she broke the news to me. She was d by her cousin. And there wasnt a damn thing i could do about it. My first instinct was to find who did it and kill them. I just wanted them dead. I was enraged, ive never felt this before. He didnt go long enough to get her pregnant or anything, thank god. But the mental image kills me! I want it to go away but it wont. It makes me furious! Im scared of what shes gona do. If i lose her i swear ill die. Shes in shock and doesnt know how to handle it. How do i make this image leave my head? How do i feel relieved? And most importantly... how do i help her?! please... i need help. I cant take this pain and anger much longer.