Why am I always dreaming of my dead "ex" girlfriend?

We met at a hall of fame thing, and since we were the only teenagers there, and we were both alone because the rest of our families were elsewhere, we hit it off pretty quickly. We ended up kissing, and kind of became boyfriend and girlfriend. Afterwards, I had to leave the country (my family was only there for my father to accept a hall of fame reward). We started emailing each other about 6 or 7 months ago, and that went on for a while, but her father ended up being killed, and due to the circumstances she thought I might have known something about it, which I didn't. We ended up fighting, and broke up. about a month later, she was hit by a drunk driver and killed. Her mother emailed me to tell me this, and said that she (my ex) had been trying to apologize, and hopefully get back together. But then, all because of a some stupid drunk, she's dead and we have no hope of ever reconciling. then, about a week ago, I've been having dreams about her, including if I was at the accident, or if she was still alive, or what if she lived here instead. Its gotten to the point where I even see her elsewhere, for example, at this moment, I can see her in my computer monitor's reflection... I'm sorry, I can't continue, I don't have the heart to right now... just please, can someone help me?