I would add to PatientMan's list to stop treating this as a defect that your wife has. If you view it that way it will become apparent to her and she will resent you for it, and that will lead to NO sex.
Your wife's desire has dropped off and she does not see that as a problem, but you are telling her she needs to seek help.
Like Mr. Bond I've been here a while and that approach to solving your problem has one outcome and it's not a good one.
You need to first ensure she's getting what she needs from you, and when she's in a good place getting her needs met, tell her that you would like her to work with you to improve your sex life for YOUR BENEFIT, not because there is anything wrong with her. Make it about you, not about what she's doing wrong.
I hope that makes sense, it's a subtle shift in perspective.
Acc
Married 18, Together 20, Now Divorced M: 48, W: 50, D: 18, S: 16, D: 12 Bomb Dropped (EA, D): 7/13/11 Start Reconcile: 8/15/11 Bomb Dropped (EA, D): 5/1/2014 (Divorced) In a New Relationship: 3/2015