I can see why you are upset. This is a serious problem that is not only ruining your enjoyment of an important area of life, it is affecting your relationships as well. We need to get to the bottom of the problem and fast.
Since you said that the problem has been getting worse as you get older, my first thought was that you have an undected health problem that is worsening over time. Your doctor checked out your hormone levels and they were fine, but I wonder whether you may be suffering from diabetes.
A common symptom of that disease is sexual dysfunction. Have your blood sugar levels been checked? Has anyone given you a glucose tolerance test?
You may also be suffering from an internal yeast infection (candidiasis). A common symptom of that condition is a loss of sexual feeling. Check out William G. Crook's book, The Yeast Connection for more information on that condition.
It has also been reported that hypoglycemia (low blood sugar) can alter sensations in the body, so you will want to check out the possibility that you have that condition as well. Many conventional doctors do not recognize the existence of hypoglycemia, so you will be wise to find a holistic physician who specializes in alternative health care practices. Make sure that you have a complete check-up along with blood analysis.
If you have diabetes, a blood analysis will reveal that. If you check out absolutely fine, then we can explore what psycholgical factors could be causing your lack of feeling. Depression can lead to a lack of sex drive and diminished pleasure.
A person can also experience a sexual numbing in response to previous sexual abuse. In such cases, the mind puts the body to sleep, and in this state a person is able to cut off from feelings and memories that might be too painful to tolerate.
And, finally, it is possible that a fear of intimacy or closeness could lead to a physical cut-off. The physical numbness serves to create emotional distance.
Without knowing more about your early history as well as your relationship background, it would be impossible for me to know whether a fear of intimacy is operating. Since it sounds like you weren't always numb, this theory is probably not likely. Unless, of course, your numbness developed following a relationship fiasco.
Many times people turn numb in order to protect themselves from the pain of another failed relationship. Does this fit your case?
I have given you a lot to examine. Please let me know what you find out. I will say a prayer for you tonight so that you burn up your mattress a. s. a. p. .