Your feelings are so valid! Currently not in the situation but thought of it before and have the same worry. I don't think I'm doing a good job in mothering or anything really so I think she would prefer someone else. But in your case it's possible she won't even like her or just another person to love her but you can't replace mom
I know this feeling I’m going through the same and honestly it’s been awful. I worry my daughter will enjoy that family unit more especially as the new partner has a child and dog so it’s literally a ready made family. I don’t have any words of advice more just solidarity! I have found that I need to keep really busy on my weekends without my daughter and have confidence in the bond that I have with her and keep telling myself that no one can replace that. X
I am soon to be in the same boat! No one is meeting anyone yet but I know it's going to happen soon and I don't know how i feel about it but I am concerned. Sorry I don't really have advice but wanted to let you know that you are certainly not the only person having these thoughts and feelings