Non-ADD partner has lost interest in sex

I've noted a wide variety of posts on this website expressing frustration with sexual intimacy when one partner has ADD.  Most of these seem to fall into two categories.  1) The ADD spouse is so easily distracted that you can't get them to begin sexual intimacy and/or stay engaged after they begin,  2) The ADD spouse (always male in this situation, as far as I've noticed) is such a novelty junkie that he spends a tremendous amount of time on pornography, leaving his wife sexually frustrated, humiliated, and/or disgusted.

My situation doesn't fit either of those categories, yet I wonder if ADD plays a role in it nonetheless.  I have ADD, and I've been married for 20+ years.  For the first few years, my wife and I got along fine in the bedroom, and I would have said everything was OK.  After a couple of children, her interest in sex diminished dramatically.  At first I attributed this to the understandable fatigue of our increasingly complicated lives, but several decades have passed since then without things getting better.  I initiate 95% of the sexual activity between us.  She goes along probably 2/3 of the times that I offer, but only rarely seems to be enjoying herself.  I don't hurry her, and pay close attention to giving her what she enjoys.  If it were up to me, we'd make love twice a week, but I feel so hurt and humiliated by decades of refusals and "going through the motions" that I only ask about twice a month.  My wife asks about twice a year, and I think I've said no only 1 or 2 times in the whole marriage, on days when I was really exhausted from work. 

In all other ways, she's a wonderful, kind, caring wife and mother.

Yes, I also express affection in non-sexual ways, such as doing favors, offering her compliments, going out to dinner together, buying her flowers, visiting her family, and taking an interest in the events of her day, job, friends, and family.  I'm not perfect, but I'm a reasonably good father.  I've been consistently employed my whole adult life.  Like all ADD males, I can walk past a sink of dirty dishes or a dusty floor without noticing, but if she asks me to clean things I will do so without complaint, although I can't promise a smile.

I'm not into strange stuff.  I don't use pornography.  I've never cheated on her.  I take "no" for an answer.  I don't smoke or use drugs.  I drink, but not to excess.

Yes, we've talked about it, over and over again, but while she always says she wants to have a happy and active sexual relationship, that doesn't translate into actual behavior. 

Does this sound like anybody else's situation, or is this not really related to ADD and I'm mistaken in trying to explain it that way?  Any useful ideas?