Ive been the same since my 2nd was born too.
I literally couldn't sleep or settle for it and I would say it spoilt things a bit. DS is 3.5 and DD is 13 months.
I still feel it but not as bad. I can actually think about other things now
.
Im hoping, like sleepdeprived I will just accept two as the right choice and move on but who knows, i've always been shocking for making final decisions then acting impulsivley
.
I think it's just watching my beautiful little bundle in a babygrow making us laugh, giving us snuggly baby cuddles and doing so many cute things that I want to preserve this forever and can't. 
I know deep down I WILL move on as I used to watch DS exactly the same way and feel dread of him growing up but you move on with them don't you.
I never (even before DD) sat and cried about the fact he wasn't who he was 6 months or a year ago so why would I with DD?
Im hoping time will calm the old hormones and broodiness for me because 3 is a whole different ball game as you say.
Your baby is only 8 months old so your hormones are very raw!!
I hear what you are saying about time but I suspect your maternal hormones are at work. You should really wait until he is at least 1 until you can really question if it's the third baby you want or if time clocks are freaking you out?
What I do is imagine getting DS ready for school, DD ready and a newborn. Strapping them all in to a car and getting out the door for 8.30am. Yip, always helps that one!
Then I take it a little further and imagine them at 15, 12.5 and 10, face tripping all of them demanding clothes irones, food, I pads and designer clobber whilst making sure im fully aware that I am infact a non entity in their lives. (is this helping)?