Yes, it's true that you won't have as much one on one time with her as you used to and things WILL be different, BUT some truly wonderful changes happen too!! Your daughter will have a sister, and seeing the love they have for eachother, and watching them play together and grow together makes your heart swell with happiness!! I have 4 children, and there is nothing that makes me happier than to see them all playing together or when one is hurt or sick, to see the concern on the face of the others. They truly love eachother. Yes, they sometimes fight, but I know that in the end they have eachothers' backs. I'm happy to know that even when I'm gone, they'll have eachother to lean on when they need to.
You will still have alone time with your 1st daughter for quite some time since babies sleep a lot (at least during the day-lol). Even when baby is awake, you can have her in the swing next to you, and be coloring or playing play-dough with the 1st. Your 1st might even be content to cuddle up with you and watch Dora or something while you doze off ;)
Heck, with 2 it's still relatively easy to carve out special time w/ each of them. When we had 2, my DH would do something special w/ one, while I would do something special w/ the other. "Special" doesn't even have to be anything elaborate- just getting that time alone w/ a parent means a lot- and if something is bothering them, that's usually when they will open up.
Don't worry, Mom, it'll be different- but wonderful :o)