Helen, I can totally relate to your feelings. We have two children and we tried for a third for 2 1/2 years and a bit, each month it was so hard waiting and then being disapointed and I was surrounded by those who had babies and for several two while I was trying!
I too felt guilty for wanting a third when I have two already! but I think if thats your desire its hard to ignore however wonderful the ones you have.
We have stopped actively trying now as it was just emotionally draining and we had to make plans as our life couldnt keep walking that path of disapointment each month. However as we havent done anything permanant about that area you never know one day I could posting just as you could that another one is on the way
Hugs to you, its a hard journey