I sympathise very much with your wife when things are getting worse. There are so many horrible thoughts that keep raising their ugly heads - feeling a burden, being "useless", how fast will things deteriorate, loss of independence just to name a few. My biggest advice is to talk to her. Tell her what's going on in your head and that may be enough for her to feel that she can talk more openly about how she feels.
How long has she been on the Fluoxetine?