Dianne,
Possibly the fact that you've been able to work through the past and she sees this, is bothering her as well and comes out in the form of jealousy, as someone suggested. Her past, along with the bad marriage you eluded to, could be smothering her happiness.

You seem fit, normal, happy, and sound to me. If she isn't, it could be very intimidating (is that the right word?) to her and she could feel even more insecure in your "happy" presence. Does that make sense?

Sometimes secure people can make others feel even more insecure by their mere presence. I have four sisters and sibling rivalry was alive and well when we were growing up. I was also the middle child...Somehow over the years, we've managed to find the middle of the road, or the good in each other. When we've argued, I find myself reaching for that "good" and reminding myself what a good person they really are, and so it softens the hurt. You might try this...