I thought exactly that free before I had a child - and to be honest, I hadn't ever intended to have a child at all, my lovely DD was a complete accident. (Do I feel I'm paying for that earlier attitude now, now that I'm actually trying to have a child, and can't? A bit, yes). And even when she was born, I told myself it would be ok that she was an only child; I raised her alone for the first couple of years, and always thought she and I together would be enough. But as she's got a bit older, and started at school (she started at her school's nursery first so has been there 2 years already), and all of her friends have younger brothers and sisters, she's really feeling the lack, I see it every day. And it absolutely breaks my heart. She has no-one else to play with at home (other than me and DP obviously, but we can't always be playing with her, and it's not the same for her anyway). She sees her friends' little brothers and sisters getting a bit older, old enough to play with her friends, and she sees how her friends are with their siblings (very protective in the most part), and she wants the same for herself. She's a very bright little girl, and really desperately wants someone of her own - and I want that for her. Not only that, but God forbid anything should happen to DP and myself but if it does, she will have absolutely no-one - and that frightens me.
I know about the resiliency of children - I myself went through some pretty nasty things in my childhood, and I've also raised two stepchildren, one who has RAD (and oh what fun that was not). But I see my darling little one, sad because she has no-one at home around her age, someone that's just hers, and it kills me. You're not over-thinking at all free - I know I do, I am accused of thinking too much daily!!, and it's probably true. But whenever I want to give up this trial, I think of my daughter, and it keeps me going. I don't know for how long I can go through this, but I think my endurance isn't anywhere near as finished as I sometimes think it is. For my daughter, I will put myself through hell and back again - if it works, and it gives her the sibling for herself that she so wants, then it will all be worth it.
Sorry, I'll shut up now. :)