Oh so sorry. I didn't realise you were talking about the situation you are in. I thought the first comment was just a thought experiment. I didn't pay enough attention. My bad.
In your case I guess she can be in the asexual part of the spectrum. One of my friends is facing a similar situation. The partner has no sex drive at all. But the partner is a great person in every other area. That relationship sustained because my friend also has a lower sex drive, but more than what the partner has.
Since this has been so long, I assume you have already tried the couple therapy and individual therapy. If not that is one thing you can try out.
But keep in mind that if your partner is really asexual, there isn't much that you can do. It's not their fault in any way. So either you have to accept the situation and build a life around this fact, or you have to move on. Since you have been in the relationship for a long time, I guess everything else is going well. Means you have already chosen the first option.
No no.. Don't blame yourself. You did nothing wrong here. Very scientifically speaking we still have no clear answer on how the sexuality of a person is determined. So far there is a consensus that there is a biological factor also in play.
It is not your failure as a partner. These are things beyond your control. She also can't do much on this. Therapy won't change the underlying reality. It will just help you to cope up with the hard realities that you are facing.
I highly recommend you take individual therapy if you haven't done so far. You may have to untangle decades of experiences to get in terms with it. It's never late, and the right therapist will definitely improve how you handle this.