We had this discussion the other night with a few families on our street who'd all gone through infertility treatment. They're the same age (early 40's), and one couple was able to conceive through IVF, and then had another child without even wanting to try almost immediately.
The other family was unable to keep a viable fetus in utero and chose to adopt instead. Today, they're completely happy with how life evolved.
In our case, we were able to carry to children full term very easily and have 2 healthy, gorgeous, thriving kids. But, a few weeks after my daughter's birth, I was diagnosed with cancer. After 5 months of chemo and 18 months of no cancer visible, I'd like another child. My husband and I are on opposite sides of the fence - I try to be thankful for the 2 healthy children I have. I missed almost the entire first year for my daughter, and I regret that along with our son's second year (my diagnosis was the day before his 2nd birthday).
My experience taught me that things do happen for a reason. It took cancer to open my eyes to what's wonderful in life and not to focus as much on what's missing.
Time is the hardest thing to pass when you want something desperately. It will lead you down the appropriate path - spend all your energy on loving your 2 current children as conditionally as possible, and you never know what surprises may lie ahead.