The Spouse Who Doesn’t Want Sex Has Affair

I was in that situation.

Initially, for several years, she pursued me romantically. I was eventually won completely over and invested in a fulsome relationship (she was my best friend).

Sex dropped off immediately, starting with the wedding night, then further with our first pregnancy and child-rearing years, further, with a house purchase, and flatlined with the second pregnancy. Other intimacy disappeared as well, and then she began picking fights. Same thing as you - complained she had no sex drive, a mismatch, and an ever shuffling deck of relationship/life issues that anyone has, with each one presented for me to solve, only to be replaced by the next one. Especially during counselling, I got to be exceedingly good at addressing her issues, including adapting my own behavior dramatically.

I later discovered she'd been having a months long affair with a co-worker (hotels, outings). And that she'd kissed yet another co-worker.
And that she'd been having an affair with a third co-worker at the time we got engaged, some 13 years earlier.
Those are the ones I know about.

We have now been separated for maybe 7-8 years and I've had time to reflect. Maybe something in what I came to believe will help you sort yourself out.

I came to realize that almost every significant reduction in intimacy (of all kinds) coincided with a significant investment in the relationship and the closing of other options. Mrs Apocrypha is a very "conflict averse" person, and wages her wars passively. She will agree to something in a moment or even suggest it, but if her heart isn't in it, she will passively ruin it through inattention, carelessness or outright sabotage and then blame others for becoming upset. She will suggest things independently that she doesn't want, in the service of what she sees as helpful, only to extract a cost later on in some other way.

I think she said "yes" to getting married when she didn't want to be married to me. So, the marriage became a prison to her and me, the jailor. I have mountains of clues pointing to this, from her actual words when I proposed, "Why not?" to the written copy of her bridal speech, in which she and her maid of honour both go on at length as to how getting married had never been in her plan (I thought, and likely it was intended as a compliment to me, or at least a "life's like that" realization). I recall multiple instances during the engagement and even on our honeymoon when she raised the topic of chucking her whole life and moving to another country for months to years on end, and if I thought I could handle that (I said no). I note that the intimacy dwindled and ultimately collapsed with every marker of permanent investment in the relationship, starting at the wedding, then house, moving, babies. It wasn't gradual, but stepped.

In short, we had a wedding ceremony, but she never joined me in what I or even "we" would have agreed is a marriage.
I invested in the relationship and our lives together, but her priority became in establishing unilateral independence.
I can see this all very plainly in hindsight, but when I was in it, I was not able to connect the dots. Or, maybe not courageous enough to push her to define the feelings behind it.

What became clear from before the marriage and after it, and during it - was that despite clinging to the "mismatched libido" nonsense, she had a very robust and adventurous sex life. It may be that exercising that sexual adventure was important to her defining herself in opposition to her associations as a married woman. While I was objectively good, giving, and game, I was simply not someone she wanted to have sex with while within the circumstance of marriage. It shouldn't have happened - she shouldn't have said yes, and in the engagement I should have tested what we had and whether she really wanted that with me, and let the consequences fall from the truth.

Once I came to this irrevocable realization (seems so obvious and simple in hindsight), I was able shift my perspective from solving the problem of sex (by making myself into whatever seemed to catch her muse), and instead anchor on the true nature of our relationship - which was something much less than a marriage and more akin to co-habitors or ex-spouse co-parents. I did not have a sexual relationship with that woman, and we would all feel better if we just dropped the pretense and proceeded from truth rather than my fantasy.