It would appear that you did everything right from the law of attraction perspective- if I understood this right you asked for some kind of relationship with an en-vogue girl, and what you got is a brief relationship with an en-vogue girl, which also happened to be very rich in your connection with her. I would say your manifestation was a success.

What happened next is that you changed your mind- you fell in love. And that's fine, there is nothing bad, or weak, about it, and contrary to popular belief it is not the chief reason for being rejected- negative beliefs are. Ironically, a term like "oneitis" perpetuates the problem- if you use it, you reinforce the idea that there is something wrong with you because a perfectly natural and healthy response happened to making a good connection with someone. And ideas that say that something is wrong with you are the real culprits in getting rejected in the first place- without them, you would most likely just be drawn to situations that harmonize well with you and rejection wouldn't be that much of an issue- there would just be different kinds of situations, but there wouldn't constantly be all that much pain on the line, because the pain wouldn't be there in the first place.

So I recommend two things:

First, clear negative beliefs. Bashar's method is very good for that: Feel your feeling of fear that you did something wrong, then ask your self: What would I have to believe in order to feel this? When you get a clear response, you're good, the belief is gone. Do this often- whenever you feel down in any way. You might be doing a lot of this for a while, then ease up a little- this is good!

Second, absolutely, keep manifesting your girl- and do keep the resistance down by not insisting that it be the one you already met, but you can certainly prefer it to be the one you already met. She's dating, she liked you, and then some guy wants "more"- that must have been very flattering for her, but it doesn't mean you're out of the question- it looks to me like she took the "safe" bet more than the "exciting" one, because she must have felt your connection too. So the trick now is to continue manifesting your girl, and also to add more detail if you want it, like that you want (whatever type of) relationship with her, and when you feel bad about that because it reminds you of your inner pain, don't shy from that- clear the inner pain using the belief method above. With less inner pain, it will be much easier to continue manifesting your girl, prefer that it may be the one you already met, but not insist it must be the one you already met.

Dating is complicated and volatile- I hope this smooths things out for you a little.