What about cousins, does she have cousins she is close with?
My son is an only child and unfortunately will remain that way as I am now a single mom and not a young one either (42), so the possibility of having a second is dwindling by the day. When I first split from my husband I was devastated at this thought (although thankful at the same time we did not have more together), but I have since accepted my fate and have embraced it. I have several friends who, for various reasons, only have one child -- we feel like we are in a little club. We have all figured out all the blessings as to why it is a good thing. Here are a few:
1) All your energy, love and wisdom go into this one little person, instead of having to split your time
2) The financial strain of raising children is a stark reality. One is better than two or more for that reason.
3) Explain to your daughter that all she has -- toys, books, clothes, etc. would be split between her and a sibling if there were one -- she would have to share, you would not be able to provide double, see point number 2! Haha! Have her realize she gets all the spoils of being the only one!! LOL!!
These are just a few I can think of offhand, but we figured out more reasons than that. She will be fine, just give her lots of love and affection and surround her with as many family members and friends as is possible. No situation in life is perfect. You just have to make lemonade out of lemons. :)
Good luck,
D.