I agree with you that @HereWeGoAgain38 will have more than enough love for both her dd and the new baby, @tsmainsqueeze - but I can also understand why her dd might feel that she will no longer be her mum's priority.
As previous posters have said, a new baby in the house means a lot of disruption, during the next, very important years of the dd's education. And a baby or toddler will definitely take up a lot of her mum's focus and attention - it is going to be a massive change for the whole household, and the dd is not unreasonable to be struggling with this.
I think that children need to be included (in an age appropriate way, of course) in discussions about major changes in their family life. When we were considering moving from Essex to Scotland, because dh had a really good job offer, we talked to the boys about it (they were 11, 13 and 15 at the time), and made sure that they were comfortable with the proposed move. We talked about the things that mattered most to them, so we could ensure that, if we did move, they would be happy - for example, ds2 was really into cricket at the time, so we promised we would make sure there was a local cricket club.
I honestly believe that this smoothed the move for them - and they felt they were part of the whole process.
I wonder if @HereWeGoAgain38 discussed the possibility of another child with her dd before it happened. I am not saying that she should have given her dd a veto on the possibility of another child, but maybe if her mum had talked through the whole thing with her beforehand, and she had had the chance to express her anxieties, so her mum could address them, it might have made things easier on the dd (apologies if you did do this, @HereWeGoAgain38, or if it was an unplanned pregnancy so you didn't have the opportunity to discuss things in advance).
I see that other posters have suggested you spend some one-on-one time with your dd, to talk it all through, and listen to her worries, so you can make sure you address them properly, and I think that is a good idea.