We're in this position.
DH - 46 - really, really struggles to cope physically with DD 19mo (he suffers ongoing depression which affects his energy). She's a lovely child: funny, cheerful, etc. Wasn't colicky or anything as a baby.
But she's so so energetic - always on the go, running about. And very, very self-composed/opinionated (has been since birth, if you can believe it - the midwives and HV remarked on it!). Everyone who meets her comments on how strong-willed and lively she is.
Because he can't manage, I wind up doing 99% of all the childcare, so occasionally feel overwhelmed. She doesn't sit still for a moment; and when she was a baby, she would not lie still on a mat or anything - she had to be in arms and carried about, seeing & doing things with me. And I can't imagine she's going to suddenly transform into a passive, quiet 3 year old.
Thus having another just feels logistically impossible: either DD would become frustrated and unhappy at lack of attention; or the new baby would be suffer as a result of DD's demands.
The thought of another like DD just is too overwhelming.
And waiting to see if she chills out as she gets older isn't a possibility, as DH will be too old to manage with another baby and the sleepless nights, etc.
So it looks like we're sticking at one.
Not sure how I feel about that at the moment. Not broody yet. But must admit, I am a little worried that it will hit me and I'll feel frustrated and sad about it.