I felt the same, being an only child I felt I wouldn't know how to share the time/love.
DD was 6and a half when DS came along, so she was an only child for ages, so I felt like I was tipping her world upside down.
DS was not an easy baby, but with the second it's so much easier, who don't feel as overwhelmed, so it's easier to juggle. We'd have one on one time when bubs was sleeping, it took some adapting but it was no where near as hard as I expected.
And DD, well she just loves her little brother-still (he is 2 now) lol
And DS absolutely adores DD, he can't wait for her to come home from school, he try's to copy her with everything.
Watching them fulfills me with such happiness. I'm so happy I gave DD a sibling, a special bond I never had.
Once bub comes along you'll see it'll all fall into place- well with a little juggling