Lord, this is a topic that has been flitting round my mind daily for the last 3 years!! I have one beautiful DD who is 4.4yo and off to school next week! I have made the decision that she will be an only, but I question that decision daily. My DH is 11 years older (I am 38)and he has been dead set against it, but has started to soften recently and we have had spates of throwing caution to the wind to see what happens! However, neither of us is absolute certain that we do want another and for us, we both have to be 100% sure and behind it for it to be a GOOD thing. I worry about her growing up an only and being the centre of attention, but being aware of it means that we can address it in the way we parent. I don't think having another child purely to give her a sibling is a good enough reason, for us.
Like Northerner I worry about regretting it later on and echo the sentiment about worrying about the child we have and not the ones we don't! I also do not think I have the energy, patience or reserves to go back to the baby stage again, which I detested! (Loved being pregnant though!) Like you, I would love to see what the mix throws up this time, but I also worry about risks involved doing it again 5 years oder - especially with the current research regarding the dads ages too.
Basically, I don't think I will ever resolve it. I would love to make the final decision and draw a big line under it, but there will always be the 'what ifs'. When people ask me if we are having another, I have started to tell them I can't as I have gone through early menopause. There is no come back to that really and half the time it is none of their business anyway and they don't realise quite how invasive the question is. It is usually just a glib throw away question to them, but always leaves me feeling upset, defensive, angry, etc.!
Bit of a long ramble - sorry, but I think most women have gone through this to some extent, whether it be moving from one to two, or four to five. I look at other women and see how effortless they make it all look and feel a fraud and a failure that I find it so hard with just one healthy and happy child. Much as I may love another, I can't see it being a good idea for us as a family!!