The short answer is that I don't know whether there's a way to get past this sudden gag reaction, or why it's happening now. I do, however, have some follow-up questions that might help. Did absolutely anything else change at the time that you abruptly went from easily swallowing to immediately gagging? Your stress levels, diet, or environment?
My husband and I have been together for 20 years. He's 55 and I'm 40. We have sex twice a month if we're lucky, and normally, I'm the one to start it. He almost always requests the same thing. He requests a blow job. But he never reciprocates. Never. If I don't, he will sometimes refuse to have sex. He hasn't done any type of stimulation in years-oral, digital, or otherwise.
My first thought, upon reading your letter, was a curious: What are the differences for your husband between penetrating your vagina with his penis and performing oral sex on you, in the context of the fact of your having given birth? My second thought was that, in your position, I would have a very difficult time calmly asking that question from a place of genuine inquisitiveness.
The problem is smell. My husband is uncircumcised, which I generally enjoy. But when he pulls back his foreskin, there is often a ghastly scent. I would describe it as mostly urine or "man-musk." I really enjoy going down on him, but this is becoming a huge barrier (and turn off!) for me. I would describe him as very clean. He showers every day. But I guess I'm just not sure how he cleans his shaft?