The fact that this occurred in such a short timespan means it's you, not her. She's stayed the same; it's not like anything she changed over the past few weeks.
Figure out why you're thinking differently.
The fact that she was your "first" in so many ways means it's going to be a very interesting self-analysis. Feel free to PM me, as well. Basically, as someone who's spent far too much time in unproductive relationships and pushed away those with whom I had a connection, I really don't want to see others make the mistakes I did.
So, this is a public forum.
The ex with whom I've spent some time lately, and with whom I might possibly ask out again... I grew distant from her because, like you, I became less attracted to her. On the other hand, the sexual attraction remained. I just started becoming totally nitpicky in a ridiculously shallow way. Basically, things were going so well that I started looking for excuses as a defensive mechanism. I went to a friend's wedding in China last September, and I resolved that I wanted a girl from another country because that's how you experience different languages and cultures... and I was still partially hung up on the Brazilian girl I used to date.
Look, you aren't going to know anyone in 4 weeks. If 90% of things are good and 10% of things are problematic, then focus on the whole; don't just hyper-focus on the things you don't like.
No one you EVER date will satisfy 100% of your requirements.
If someone's 90% of the way there, then work on bridging the gap. Someone who "gets you" and who you can be yourself with... that's rare. I was never myself around my fucking hot Brazilian model girlfriend, and honestly, she was never a match for me. Don't overvalue physical attractiveness to the point of undervaluing being your true, authentic self with someone.
Why don't you see a future with her? Be specific. And, by the way, "it's 4 weeks in, so how can I think about the future?" is perfectly rational.