sometimes for whatever reason adults divorce. it's usally stemming from a mutual agreement when they have been married longer.
when i read your post the first thought i had was at your age my father was dead-he was 45 when he died. i had 2 younger sisters that were in elementary school. so while your parents are not together anymore at least you have both of them to enjoy. rather than struggling with something you can't change, try to look at the positive, you have both parents living.
i'm sorry this has happened to your family and clearly this is troubling you. it must be difficult to understand...i don't know if you know the circumstances of their decision. talking about your grief will help you so i'm glad you posted. but hope you will see that many of us didn't have the joy of being young adults having both parents. to me that is a bigger loss, imho. it doesn't lessen your sorrow but i hope it will give you a more positive perspective. i'm sure you wish that both your parents be happy. if they were unfulfilled being together still their decision to part was a good one.