I was totally there. I have a 2 year old and a 2 month old. I remember crying when I found out I was pregnant. Not tears of joy, but tears of guilt. I felt like my first daughter's life was going to be turned upside down and it would be all my fault!
Like someone else said, the minute my second baby was born, the flood of love and emotional attachment to her was overwhelming, unexpected, and incredible. And even if you don't have that, everything does work itself out. My first daughter is on the sensitive side, and takes awhile to transition to change, so I was feeling awful the first month after bringing the baby home. That first month, she never ever had a problem with her new sister...she did, however, express some anxiety over the fact that I could not pay attention to her as much as I used to. Some great tips I got from other moms that worked:
1.) have "breastfeeding toys" (or "bottle feeding toys") - when I'm at home and bfing the baby, I had designated toys that I knew my older one loved (kai lan doll, microphone, play doh, to name a few) that she could only play with while I was nursing. This made nursing the baby a special time for her, something that she actually looked forward to. 2 months later, I no longer have to bring out the breastfeeding toys.
2.) when the new baby is napping, spend all your time and energy on your older daughter (when she's not in preschool). do something she loves together. resist the urge to clean/cook/organize (make your frozen meals now). they do notice and cherish the effort.
3.) if she does express some anxiety, talk it out with her and affirm her feelings (eg., "are you sad because mommy can't play with you as much as she used to?")
Keep in mind my daughter was 22 months when her sister was born - an age where their sense of security is not 100% yet. Generally, 3 years of age is when this sense of security and independence sets in, so I bet your daughter will do great! I'd also like to note that 2 months later, my eldest daughter wakes up asking for her baby sister DAILY. She hugs, holds, and kisses her constantly, and can't imagine living without her. Her anxiety regarding our relationship is completely gone. You are giving your older daughter one of the greatest gifts of all - a sister and friend for life! :)