Well, I don't necessarily think she could be obsessed with you. maybe she is just madly in love, or at least she thinks she is. just tell her that you don't want to be with her, that you've moved on. explain to her why you don't want to be with her, don't leave her wondering. tell her you're happy now, with your new girlfriend. it may break her heart but if you don't love her and you tell her that, then she has the chance to find someone she does love, because everyone deserves to be loved.
zenonkanyane