Worries and guilt about having a second child on the way.

I'm feel so sad or guilty that I'm cutting my daughters time as a only child so short. Maybe its because I was a only child for 9 years before I became a sister. My first pregnancy I didnt have these worried feelings at all. This time I'm scared about how things will change like if my daughter is ready to share us with her brother, making her a big sister at 21 months, if I can be there for her as much as I am now. She knows she is loved and is my everything. I don't want that to change. Yeah, we will make things work and life carry on. I just wish to be less emotionally stressed out about it. Did anyone else experience these worries and were they unnecessary?