Is it normal for feelings to just randomly die?

I know, that's the sad truth :[
But, yeah, that's basically what it was. But it's more because, before her, I never had any luck with girls. I was just so lonely that when I met a girl who liked me, I kind of lied to myself and told myself that she was so great. But really, she was boring, cold, and perverted.
I found sex disgusting before, but it seemed to grow worse as I began to know her. And, most asexuals don't involve themselves, but some can. I personally can, because I suppose in the heat of the moment, it's exciting, but afterward, I'm disgusted with myself, and everyone else who acts sexually.
And, I think she understands that I dislike sex, and that I don't enjoy her company much, but she counters it with anger and jealousy, which only makes it worse.