Its the excitement of something different. You and your D are love, love is pain. I believe that they block out love, as they have blocked out pain all of their lives.
What they find in the OP is "JOY", like at christmas, but everyday with the OP is christmas. They eventually wake up, realise its August and then want to come back to you.
You need to make a decision as to whether you want her back, if and when that day comes. Until then, you need to soak this all up. GAL and cry, cry and GAL. And keep posting!
Just cos things are going right, doesn't mean that they were always wrong.