OK, but all of a sudden you expect her to do a 180 after 18 years? Your wife sounds like she's a bit shy about initiating. Maybe 'being the pursuer' makes her feel dirty - who knows what's going on in her head... DO YOU?
Talk more about this with your wife. Never, ever keep any secrets between you.
Tell her just what you've told us here. In fact, print this thread and give it to her.
She'll either jump you right then or kill you. Either way, problem solved.
It may be useful to know that the twenty year time frame is commonly a bad period for marriages. There is a statistical spike in divorces around 18-22 years, after many years of low-risk. So, you are not alone.
The other thing to know about this is that after weathering this rough patch, most marriages become more rewarding, closer, and more satisfying. So I would encourage patience, with yourself as well as her, try some of the things others have suggested while using your own judgment and knowledge of her, and find things each day for which to feel grateful. It sounds like you have a lot.