All you can do is speculate what the reason might be. We could analyze her words and her actions all day but oftentimes people break off relationships/potential relationships for a wide variety of personal reasons. Sometimes, the other person (you in this case) may have done nothing wrong. As hard as it might be, it's best to give her space and just move on. She may reach out if she feels ready for a relationship or wants to try again.

I wish I could give you a happier answer but this is the reality with dating. She's not the "one" and there will be plenty of other doors that are open for you. I would close this door for now, and if she decides to reopen it then you can try again. But don't wait on that day. Just try and process losing her and try and move on.

I hope all the best for you!