The one thing in your letter that jumps out at me is that this girl says that your feelings are too serious. She is telling you something that is very painful for you to face, but you must face the truth of her words or else risk being even more hurt down the road. She is telling you that she isn't as serious about you as you are about her.
I know you care for this girl and that it hurts to face the fact that she doesn't reciprocate your feelings. At this point, you need to have the courage to ask her to tell you more about why she thinks you're too serious.
Is it that she doesn't feel the same way as you do and she wants to let you down easily. Is she afraid to become more involved than she is and is she trying to cool you off toward her in order to help herself stay distant? Is she trying to keep the relationship cool because you are physically far apart?
Bite the bullet and ask her to spell out where she's coming from. When you know, you will have a much clearer sense of how to proceed. If she's protecting herself because of the physical distance, that problem can be solved! If she doesn't have the same strong feelings as you do, that is harder to fix!
Feelings can grow stronger over time as your level of intimacy and attachment develops. So, if she says that she doesn't feel as strongly about you as you do about her, then you want to ask her if she thinks that she could grow to care more for you given devotion and patience on your part. Gather your courage and talk.
You sound like a wonderful guy who is capable of a lot of love and caring. You deserve to get back as much as you give!