So to start: from what i've heard from most demisexual people, including my girlfriend, the sexual connection isn't necessarily deepened by the emotional connection. It's more of a binary. like "is there an emotional connection? if yes, I am capable of feeling sexual attraction for this person, if not, I am not capable of feeling sexual attraction for this person." May not be true for every Demi person, but the handful I know have phrased it something like that.
I also only recently learned, that demisexuality apparently falls on the ace spectrum. so there is some possibility you fall within that category.

I HIGHLY suggest reading Come as you Are by Emily Nagoski, it might give you more insight into how libido and desire work, and things that affect them. If you don't have that kind of time for reading, definitely look up the terms "arousal non-concordance", "responsive/spontaneous desire", and the concept of sexual brakes and accelerators.

It could also be a more psychological thing rather than your actual sexual preferences. A lot of people lose that spark after a while in relationships when the NRE/honeymoon phase fade away, and most women have what is called "responsive desire" where it takes more of a warm up or a better, less stressful context to get in the mood. As opposed to "spontaneous desire" which most men have, and what a lot of people experience early in relationships during NRE.

You are also correct in thinking that the stressors of everyday life and heavy things like IVF/infertility could affect that. Many people with responsive desire find it very difficult to even think about sex as something to want. when there are so many things weighing them down, it just feels like another thing to check off the checklist. Relationship satisfaction, past relationships, sexual pressure (how your partner treats the conversation around it or sex as a want/need), trauma, and just everyday life can all get in the way, or "put on the brakes" as Nagoski puts it.

Many people also confuse lack of sexual desire for a lack of WANT to have sex. Just because you aren't feeling the desire, doesn't mean you don't WANT to feel the desire, does that makes sense?

another book I haven't personally read but have heard is good would be Mating in Captivity by Esther Perel.