Seeking expert counseling is a sign of strength. A personal relationship with a caring professional is proven clinically effective.
There are many possible answers to your question.
I shall name the most common reasons and then if you wish you can get back to me and we can expand in the area of the probable reason.
1) Perhaps you suffer from erectile dysfunction and cannot get or maintain a full erection. This can usually be helped with ED medications such as Viagra, Levitra, or Cialis.
2) Perhaps you have a low libido, which could be caused by low testosterone levels. There are tests for this and booster shots available.
3) If you are using certain medications, particularly antidepressants, they can significantly lower your libido and sex drive.
4) You may be suffering from depression, which often inhibits interest in sex.
5) You may have lost attraction to your wife.
6) She may have lost attraction to you
7) She may experience pain when having sex and thus avoids it.
8) She may be on antidepressants or be depressed.
9) Either one of you might suffer from some other physical ailment that prevents you from having a good and frequent sexual relationship.
I suggest that you both have complete physical exams.
To rule out physical causes.
If there is a relationship problem, I strongly urge you to find and make an appointment with a sex therapist who is usually a mental health therapist, licensed, with additional training and certification. The best professional organization and list is AASECT. Here is their website. Go there and click on your state on the map and the list will appear.
AASECT - Member Directory
If I can be of further assistance, please to not hesitate to get back to me. May goal is to help.
I shall keep both of you in my prayers.
Elliott, MAE, LPCC, NCC, CCMHC