What is my problem and what do i do about it?

Im about 18 years old and in prime physical condition. I hate everyone except for my girlfriend whom i am marrying. I get extremely angry over stupid situations and the only things that make me happy beside my girlfriend and music about hatred and suicide are thoughts of death and mutilation. I know this isn't healthy but i am far too prideful to seek extensive help. My girlfriend called my to attention about how "dark" i am and she says i should be happier. She knows that even though i often think about harming myself that i never will but it is also understood that if for some reason i act through my anger at something that that thing will be broken or i will, this causes her to shut down to me, which i find fairly frustrating because i don't want any further harm to come to her after what she has experienced earlier in life. She just wants me to be "happy" but i believe that is a dead emotion within me. In fact, my only hope for emotional release is within the army. I take time to specifically detail disembowelments in my drawings, schools have always said im "emotionally disturbed" and "autistic" "moody" "bipolar" and much more from kindergarten on. Im convinced that aver so long of a routine that this is who i am but i still don't know. My own father even tears me down telling me i fit the bill of criminals who when they decide they wish to die open fully automatic fire on a crowd before they commit suicide. I feel as though i will destroy my own relationship from emotional discharges, some of which i black out yell at my girlfriend, she forgives me but it makes her more distant. Even though she says i am the only person she ever wants to be with for the rest of her life i find it hard to believe in between my actions and other guys flirting with her, which actually makes me feel proud of myself but i find it extremely frustrating when i don't know what to do and by the end of the day make a total *** out of myself. What the hell is my problem and how do i resolve it, for the sake of possibly my own survival? (Hypothetically being hunted)