Jez,
We share the same story. I have 3 girls: 12, 11 & 9 years old. Hubby always wanted to try for a boy. I was so done with having babies I can't even tell you!
When I found out I was pregnant I cried and cried and cried. I knew God was responsible for this little creation but I was afraid because of my age (I was 36) , didn't want to start all over again and life was changing all over again. Not happy.
Everyone told me that it was going to be okay and they were really supportive and listened to my fears and my tears.
All I could say is that this baby BOY was the biggest blessing to our family. I had to go through many heart changes and believe me, it was tough. But the miracle of his addition to our family and brought so much joy to our lives and my children absolutely adore him.
Your pregnancy may be different because you are a bit more mature than you were when you had your other children. But, you ARE more mature and that in an of itself is so wonderful. More patience, more wisdom, it's awesome.
Blessings to you and keep me posted. I would love to be a support to you during this time as I truly know the roller coaster ride of an unexpected pregnancy!