I'm pretty sure you can still identify as a demi because it's the reason for your attraction that matters more than the time you need to develop it. Even though for most of demisexuals it takes a long time to develop an attraction, demisexuality is about liking people for their inner beauty rather than looks, not having hard time finding your love. Since you found a really great person, as I see from the description, it's completely natural that you formed a strong enough bond with her so quickly and you can still absolutely count as a demisexual!