I felt the same way when I was pregnant with my 2nd child and was so worried about my relationship with my first son. I have 2 boys ages 5.5 and 2.5. My husband and I waited a long time to have children and when we had our first son I simply loved being his mom! I walked away from a great career & proudly poured myself into my new role as Connor’s mom!
My husband traveled a lot for business and me and my little boy spent tons of time together over his first few years. My husband & I had talked about having a 2nd child, but I was reluctant as I adored my relationship with my first born. Like you, I wasn't sure I would have room to love another child or that I wanted to mess up the relationship with my oldest.
The first few months were hard when my 2nd son arrived, but I could not believe that I could love my 2nd son just as much as my first. But guess what? I did and I do! It’s so fun now watching my boys play together and we could not imagine our family without our youngest.
You sound like a great mom and your new little one will find his or her place in your heart, schedule and with your daughter. Try no to be too hard on yourself as emotions are part of the process of adjusting your mindset to a new normal.