Don't worry I dithered for a while before committing a second time. You're not selfish for thinking hard about it. I waited until DD1 was 16 going on 17 before I went through approval again. I knew one child wasn't enough for me, but I had so many worries..I couldn't imagine loving another child as much as DD1 (which sounds really stupid now that I have three, and love them all as much as each other), and I worried I wouldn't be able to give DD1 the attention she needed, and I wasn't getting any younger and wondered whether I had enough energy! I knew I wanted another older girl, but I knew what I was getting into this time around, and worried whether I would take on too much. The worry about money, and space, and all that lot!!
I know you know this already, but only do it if it's really what you want, not just because of your DD. It can be lovely to have a sibling, but sometimes it isn't so good, so it's wise to take a while to think on it before doing anything
I certainly don't regret doing it a second (and third) time, but DD2 is definitely the hardest of my three to parent, and it was a big shock to the system to have another at home, even though she did not move in till DD1 have left home! Your concerns are all valid, and not selfish. Adoption is a whole family decision
It's good to here you're so happy together though :)