Well sometimes you just need to be blunt. I am not asking what are the causes you're not dating her, since you've mentioned that you already love her. And she loves you back. But i guess, you have some problems, family/soceity/friends of your group, and much more for which you are not dating her.
She waited for 8 long years, don't give her fake hopes anymore. Make her understand, and i don't think you two can ever be friends, since you truely love each other. But if you don't want to date her, because of the circumstances, which i mentioned earlier, then just tell her. Her heart will be broken either way, if you tell her now, or 2-3 years later. But whatever you do don't delay, if you slightest bit care for her, and i guess you do since you two are best friends. Offtopic, i wanna say Just think for a bit, if you want a girl, who is loyal, and matches you're type, she might be the one!