S.H.
I felt like that too... when I was pregnant with my 2nd child.
My eldest at the time, was 3 years old.
It is a good age, to have a 2nd child.
It was for me, although it was not planned that way.
When I had my 2nd child, I STILL had all those great feelings about my eldest, and then with my 2nd child.
Don't worry.
There is plenty of love to go around.
A Mommy's love, is not finite or constricted.
It just grows.
And you will, do what you have to do... all over again with another baby.
My eldest, LOVED having a sibling.
They are very close.
2 peas in a pod.
And it is wonderful, to be a family of 4.
One thing though: when I was pregnant with my 2nd child, I spent 'my' pregnancy, as "prepping" my eldest, for having a sibling. SHE was having a baby too. I spent TONS of time on just her... so that she transitioned and adapted well, to having her baby brother.
So by the time I brought my 2nd child home, she was not all shocked about it and she still knew her place in the family. She was secure about the whole process. I explained to her, what will happen, what a baby does, that I nurse him, that he will cry and wake. That she is STILL, "my first baby, always." I spent lots of time, explaining things to her... and she had 'bonded' with her baby brother already, while he was still in my tummy.
I ALSO explained to my Eldest, that as Mommy's tummy grows, I will need to rest and nap. And we would do that TOGETHER. She understood. I also explained to her that as Mommy's tummy grows, I cannot carry her or might not be able to move fast or do things... and that going up the stairs, might be hard for me.
She then, really understood. She was a real trooper. Because, I explained things to her, in a way she could understand.
I also let her, express any feelings she had or any questions. I told her I am always there for her. I do NOT expect her, to suddenly act, all grown up... because I know, she is still a little girl, too. Even if she will be having a sibling.