I'd say it's too early to say something deep and meaningful to her. Of course you can thank her for the wonderful times you've had these last few days, but it's way too early to give some kind of "I love you". She might be totally amazing but wouldn't you say that it might be more infatuation or lust at this point? Besides, if she KNEW that she was only here a short time, perhaps she had it in mind as a fling. But it might be to easy to call it a "fling". Maybe she wanted some great guy to enjoy her time here and have good memories about.
I'd be open about how much you've enjoyed her being around and ask her if her intentions were to just be a one time deal, or if she'll be back and want to keep ties with you if you guys were compatible. (sex or otherwise) Be open to her first about that you're COMFORTABLE with whatever her ideas are because you had fun no matter what. If she feels that you're PUSHING for a specific answer, she might be afraid to say what she really feels.
It sucks that she's taking off and you feel this way, but it's not so bad because taking the time to get to know someone over weeks and months, is a good thing. There's probably tons you don't know about her, good and bad so who's to say that there would/can be a long term potential for her.
Even if she does leave and that's that. Aren't you glad that you had a great time and she rocked your world? (that's how you make it sound) I'd rather have enjoyed and lost then never had that opportunity at all to have a great time.
Like one of my ex's I had the best time just being me around. It didn't work out for various reasons but for about 5 months, things were just amazing. I felt like we were made to find each other. And maybe in the grand scheme of things we were. But not to last forever. I learned alot about how women are and probably more about myself thanks to the hardships (and some good exp too) that I went through with her. But I don't regret anything about my actions. All I know is that I treated her great and I felt great doing that.
If you're happy with how things have been and that you've been able to just be you and share these times then be happy for that! There's so many times that people just don't want you to be who you are or try to change you. God knows I tried changing people when I knew it wasn't right. (and that I changed for people because they wanted me to be someone I wasn't!)
Anyway, the easiest(and hardest) way to look at it is if it was meant to be...it was. And just accept the things you can't control, which is her choice whatever it may be.
It's great to have strong feelings for someone but remember that YOU come first. Don't let your life revolve around her and don't ever say that comment again about "she could do so much better than me". You should ALWAYS believe that you're just as great as anyone else if not better. Plus women love confidence.
Good luck!
3 out of 2 people have problems with fractions.