Is she totally sure she is asexual? I don't mean to sound patronising but you are both fairly young and have only been in a relationship with each other since your mid teens.
I wonder if simply she is sexually bored and doesn't realise it? I only say because when I was of a similar age I went through the same thing and questioned whether I may be asexual, turns out I had, in fact a very healthy libido I was just not interested in sex with him.
I think she might be so hard wired in Monogamy that she might not be able to separate her sexual boredom from you, from her love for you, therefore she interprets it as not being interested in sex at all, when in fact if Daniel Craig was in the room, it would be a whole other story. It could be that once she gets the hots for someone else than similarly her emotions will go with it in a traditionally serial monogamist way?
Either way from what you have described I think the relationship is nearing its natural end.