try not to worry too much ... I really feel like this is one of those transitions in life you can't really prepare for as much as you just have to take one day at a time and figure out once you're in the situation ... but short of severe ppd, it WILL work out and it WILL be okay
My first dds are 21 months apart and my second set is 22 months apart. First to those of you about to embark on three - let me say that from everyone I have ever spoken to, going from 2-3 is a breeze and so much easier than going from 1-2.
1-2 was my hardest transition, but that said even, there were more days than not when I really felt like "this is easier than I thought." And I don't think I was nearly as prepared as you are Juliwan. I was pretty naive and excited and not giving much thought to how much #2 would change things, just excited for her arrival and in an idiotic bliss about the whole thing, kwim?
I think one thing you need to mentally prepare for (and it sounds like you already are) is the "wedge-effect" I don't think dd1 really felt it, but after a couple of days, I was a little resentful of the wedge that dd2 drove in my relationship with dd1. Before dd2, dd1 was my whole world (and visa versa) and then once I had dd2 and was nursing 24/7, everyone kept coming to entertain or take dd1 on little outings and I was stuck with the nurse-aholic crying baby. I felt like screaming, someone take this baby and give me my dd1 back! But the upside to that was 1) I really don't think that it affected dd1 negatively. She really developed a much closer relationship with dh from that point on and now they are practically inseparable. 2) It really didn't last all that long, perhaps the first month or so while I worked out the kinks...
Since you mentioned the nap thing, I'll just say IME - it's heaven having an older sibling still be young enough to take one
You can usually lay down with your older sibling and baby and even take a nap with them
I mean especially that first month when baby will be sleeping like a newborn, it shouldn't be a problem to get them both down at the same time and I remember fondly napping with them when I had only 2 kids. Once I had three, the oldest wasn't really napping anymore and when I did get the little ones down at the same time, I felt like I should really spend that quality time with dd1, yk? So I didn't nap as often having #3 or I'd have to get really tricky and scheduled and try to get the younger two to nap while dd1 was at preschool AND make sure if I fell asleep I'd get up in time to pick her up!!
I totally understand your stress. It's a huge unknown, but I really think that having your kids closer in age like you will (+/- 2 years) is ideal. It's definitely tougher in the beginning, but as they get older, you more than make up for that struggle with IMO and IME less sibling rivalry actually.