Hi roseberry and seasalt it will be fine! I remember feeling exactly the same in the lead up to giving birth to number two. I am now feeling the same about how my younger one is going to cope and how I am going to cope too!
When DD2 was born things were much more fluid than with the first one, as she had to fit in with the other things we were doing. When I was breast feeding, I had a large craft box for DD1 to do, I read her stories, and she watched way too much tv. She did have some points when she acted younger than she had been, and I let her do it, and treated her in the way that she was obviously asking to be treated, I used to tell her that I did all the same things for her when she was little, and if she still wanted me to do them then this was fine, but the reason I no longer did them for her was because she had learned to do them herself. So when DD2 stopped breast feeding and had a bottle, DD1 had one too (but only a few times).
The times I found most difficult were bedtime, as baby had to come too, and trying to explain why the baby was staying up later than she was. Also at the time DD1 was getting up at 5.30 to 6am every day, which was a trial when I had been up half the night with DD2.
It's busy, and constant, but you will both find ways to make it work for you. I quite often put DD2 in the pram when it was time to sleep, and pushed her to the play park, once she was asleep, this left DD1 and I with time to do things together, so nap times became her outdoor times if you see what I mean.
Yes I did get annoyed, and had to do a fair amount of apologising and tell DD1 that I got annoyed because I was tired, and that I should not have done.
One of the main things I found was that DD1 looked huge to DD2 and it was very easy to forget she was also small (not that small as she was 3.8 years when DD2 was born) and still needed things done for her.
Nowadays I feel quite lost if I just have one child with me, and it seems more difficult than with both of them there.