I have been having these exact same thoughts. Ds is 27 months, and we are planning on TTC the 2nd starting in October. One thing I have to keep remindg myself is that everything with babies and toddlers is temporary. I know it doesn't feel like it at the moment (especially when it's being woken up 12 times during the night), but it really does end. Sometimes it gets worse temporarily (we went through a phase of being up at 5am!), but it always eventually gets better. While my ds still doesn't sleep through the night, despite being nightweaned, his waking is already so much better than it used to be. He used to wake up every hour until I came to bed, and then every 2-3 hours after that. It was driving me insane! I would lay there with a nipple in his mouth, fuming and feeling sorry for myself, and thinking about how there is no earthly way I could add a baby to this mix. And while I'm still not getting 8 hours of uninterrupted sleep, our nights are so much better than they used to be. And it will still be at least another 9 months before adding another baby.
Dh and I decided to just go ahead and go for it, having faith that all of us will adapt and make it work. Ds is all of a sudden maturing so quickly - wanting to do stuff for himself (pour his drink, go down the big slide, put on his jacket and shoes), communicating better and being reasonable about things that he can and cannot have, etc. There are some days I feel like he doesn't even need me anymore!
I am still really scared at the idea of having a 2nd baby, but we know we want at least 1 and maybe 2 more. Even if you got pregnant tomorrow, a lot of things change from 8.5 months to 18 months. Ds is so different than he was 9 months ago.
And, I watch him with other babies - my neighbor's baby (who is 18 months younger than her first), my neice, etc. - he is so sweet, gentle and curious about babies, that it makes me want to have another one tomorrow!
It may be that you just decide to wait longer than some other parents do. Our ds will be just over 3 when our next baby is born (if all goes according to plan), and while I can definitely see advantages to having them closer in age, I just personally wasn't ready for the 2nd until now. Although don't misunderstand me - I still am not totally confident that I can do this. I am just leaping forward on the faith that our little family will adapt and it will work out just fine. But I do feel much more comfortable now than I would have a year ago.