Had a situation similar but not as in depth or long lasting. Our frequency took a dive after about 2 years of marriage, we were both early-mid 20s with no kids or health issues. I would get angry about it and we'd fight, she would just chalk it up to her not being in the mood. For whatever reason. We were stressed more and more and it got worse and worse, to the point where we'd have sex maybe once a month, and once a week was probably the max for about a year, maybe more.
Finally got to the point where I resented her a lot, I didn't want to come home from work, and when I was home I was always just pissed off with a short temper. I couldn't sleep, wasn't rested, and that added to the cycle. I broke down and said that I was just about done, and she asked if I wanted to sleep with other people. I replied I didn't, but I damn well wanted to sleep with someone and while I much prefer it be her, it would be someone else if she wasn't willing. We ended up going to a therapist and it took a while, but eventually we got reset and she learned not to carry her work stress home and I learned to communicate better. Things got better and we're perfectly happy.
Our frequency depends on what's going on in life, it's been 2 months because our daughter is 7 weeks old, but it should go back to normal (or the new normal) fairly soon. But we're both better at making an effort. She still shoots me down quite often, which has led to me not initiating almost ever, which she hates. But fuck that, if you're going to shoot me down all the time, I'm not going to waste my time. We still need to improve that.