Not selfish, just in shock and awash with pregnancy hormones. With my first it felt so right to be pregnant, with my second - also much wanted, but taken by surprise - I worried that I was making a terrible mistake. That feeling eased over the months, it did not last.
You dd will cope marvellously with the changes ahead. It is so exciting to have a baby, and she will be old enough to get involved.
Take things one little step at a time. You don't need to talk with your dd about the pregnancy yet, you can wait until you get your head around it first. AN depression is not hugely unusual, and you can talk to your GP about it, or your HV if you prefer.
Make any changes to your dd's life earlier, so she had a chance to get used to anything new well before the baby is born - if she has to change bedroom, for example. Let her know that new babies can be quite a pain: they cry and they poo, and they can't play games at first, but they do become more fun as they get older.
As for love (I know you didn't mention it, but many people think it), don't worry about that either. There is no limit to the amount of love a person can feel or express. You will still love your dd just as much after the baby is born, if not even more, and you will have just as much love for the new baby as you had for the first.
My eldest was 2 when dc2 was born, so a little different to you, but when I was pg I got him used to sitting next to me on the sofa with a book on a cushion on his lap when I read to him. That way I could still read to him wwhile I was breastfeeding the baby. He got to feel that feeds were often his special time because he got to have a cuddle and an uninterupted story.