Don't expect too much, too soon...take it one day at a time.
Bella was my only

girl for over a year till I adopted Abby thru YTNR and it was a little overwhelming the first couple of days for all of us...but, it did get better!
You might want to try involving them both in some kind of playtime or a short walk around the block...create a routine of doing something all together to help foster a relationship between ALL of you.
This year, I adopted Phoebe again thru YTNR and it was much easier to introduce this little one into the pack.
I also began fostering for YTNR so, every once in awhile...there is a visiting little

one until they go to their furever home and all the doggies do really good.
Just try to relax & take it easy and your little one will soon realize that you're not replacing her but, adding more fun to the mix!
Good luck!