If you want to obsess over her, go ahead!
The places you went with her, her hobbies and interest, the songs, books and films she liked, the clothes she would wear, the smell on her skin, softness of her touch, colour of her eyes, sound of her voice, her beliefs, fears and aspirations, the history- remember it all.
Hold onto every detail, write about it, think about it, dream about it.
You might never move on, but why should you have to? This is what society expects from you.
Doesn't matter who else you date, fu*k, marry or have children with. She was your first love and you should not forget her.
Embrace the misery, let it beautify you. Soon you will find comfort in it.