I hope you dont think that I am insensitive but I have a opposite problem to you.
Firstly I was an only child and had a wonderful childhood, I wasnt spoilt at all and mum often says the house was full of children. She came from a large family and wanted lots of kids, but she had quite severe health problems after having me so it was never to be. DH's sister is 13 years younger than him, so he was a only child for most of his childhood too.
We had a really tough time getting pregnant with dd1, 3 lots of IVF and one eptopic pregnancy, so were so grateful to have one healthy daughter and were not expecting any others. Didnt use any contraception and guess what, 8 months later DD2 arrived.
Anyway, thats beside the point, I can honestly say that I had a wonderful childhood, may be a bit harder work for the parents, and probably not what you imagined your life would be like, but please dont worry.
As crazy as it seems, I was really worried when our suprise came along, I had conditioned myself to only expecting one child, how on earth would I love 2 children, I just couldnt see how I could do it, especially as I never had to share my parens, but its all worked out well in the end and I'm sure it will for you too,