Hi Mittel!
Just wanted you to know you're not alone, I feel like you a lot of the time.
I vacillate so much - fundamentally I know it is a good idea for us to stick at one for MASSIVE reasons - a disability that DD has luckily not inherited but a future child might; money; time; and the fact that I don't actually think I want another!
But then... I worry about DD. I worry that she will be lonely. And I worry about some time in the future when all we will have is her - for example I think of my parents who have 4 kids and 5 grandchildren and their lives are massively enriched in their older age by all the visiting and activity - I worry that our lives will feel veyr empty when DD is grown up, with just having had her, and that if she for whatever reason doens't have a familyy of her own it will be a very small lot of us indeed.
I know this sounds silly as you could have 4 kids and all of them oculd move away and/or hate your guts and so you might never see them!!
Do you ever think that maybe part of the reason for you feeling like this is because, at her young age, she doesn't yet have a massive 'social' life of her own... ie my DD is only 2.5 and isn't even at nursery yet, so the circle of other kids for her to play with is TINY. However when she is at school (hopefully, anyway) she will have loads more opportnities for socialising with other kids, making preopr friendships etc...
In fact I do occasionally wonder whether some people have a second child when their older one is a toddler as they feel like you and I do - they feel the toddler is lonely - only to then have the siblings spend the next 18 years off with their own friends and ignoring each other??!
Good luck OP, I feel just as you do and at 39 time is running out either way.
Both my head and my heart tell me we are right as we are (and phenomenally lucky) but I do have my moments!!