Hi, I had my children early, but my daughter is now 40 with no reasonable prospect of having a child. I know she would like one but only if the circumstances are right, and that's not the case.
When she talked to me about this, I told her I had every faith in her coming to terms with it because she's a stable, sensible person (with a great mother!), who'll process the pain and come out stronger. There's actually a strong tradition of single childless women in my family ranging from my great-aunts, my cousins and nieces.
My sister, a doctor, told me she sees a lot of women around 40 who've realised they won't have children and go through a period of depression or sadness until they adjust.
Fiona, you sound like a strong person who can make the difficult decisions. If you don't end up having a child I'm sure you will find a way to fill that part of your life, whether its nieces and nephews (a single aunt is a wonderful thing for children) or travelling, or getting involved with children in some other role like sponsoring. There are many great options.