Being in love with someone is the sweetest feeling in the world, just before it turns sour, and all of the butterflies in your belly become gargolyles at sunrise. When you love someone with all you've got only to realize at some point that that love is not reciprocated, or has stopped to be, you would only have begun to know the several shades of pain.
You would try to drown out the pain with comfort-food, leave your hair disheveled and not bother with a bath; you'd listen to break-up songs that tell you you are better off without them and you would sing along in a slightly-hoarse off-key voice and want to believe the lyrics, really want to believe it, only to wake up in the middle of the night and burst into tears because it's all useless. You miss them, want them beside you, and it kills you to know it would never again be that you would hold them, caress them, kiss... and it hurts so so much...
Then you become angry, and this is a very seductive phase because anger makes you feel a false sense of power, it gives you something to look forward to thinking you could somehow get back at them for walking all over your heart, and it is a more functional state in which you can get things done fast with all your rage as fuel as compared to sulking in self-pity and tearing out wads of tissue. The angry stage is so seductive it not only lures you into a cave but keeps you in that cave where you find purpose in knowing that person who has betrayed your trust would one day know your wrath. You hold the thought and there is direction in that singleness of purpose. Anger becomes a lifestyle, and like a cancer it begins to spread, finding other targets: your family, friends who are worried for you, colleagues who try to hook you up, people who say they would like to know you better, and ultimately: yourself.
Anger makes it possible for you to push people away and think its the smart thing to do. Never again am I going to be hurt, you tell yourself. Never again would I let anyone get the best of me. You convince yourself you are happy -"I'm as happy as I can be" -and it doesn't matter what anyone else says because they don't know what you've been through, they don't know how you feel, how you never want to feel again...
True, anger is only a natural consequence of feeling betrayed by someone dear. It is perfectly natural, perfectly human to feel rage at someone who just broke your heart -and there's really no explanation that can make this right -but when your rage grows from kicking tyres and smashing furniture to building walls of resentment and distrust you would only have handed out the reins of your life to someone you already know no longer cares, and caged yourself in.
Yourself: that's the only person you're hurting, and yes you'd like to think otherwise, anger can be tricky in that way... it's alot like drinking poison and expecting someone else to die... Staying angry would only make you push those in your life away, and keep others from coming in. You're going to keep hurting, remaining in one spot while your excuse, your mask for self-hatred remains blissfully unknowing, living a life while you just exist.
When you've been angry for so long it can't be easy unburdening, but it'd help to realize that the only one you should live your life for is yourself. Love life for it's worth and not in the hopes of getting back at someone no longer dear, and this is only possible when you realize there's only one person who owns your heart: YOU.