Hi The_Caretaker, sorry to hear about your situation. I agree with what GuitarRox wrote. I personally think the old adage "use it or lose it" is not true. I have not seen that in my own experience, and also I do not see any physical reason why this would be true. Erections often occur when you sleep so it's not as though no blood is going there or it's never fired up if you aren't engaging in sexual activity.
Perhaps letting your lady know that there has been a change in your functioning down there, physically or you suspect physically, so it's not really fair to compare a time when you got your ex wife pregnant with now, and perhaps if you find some articles or information on how physical issues can affect erections and share one of those with her that might make her see that there is more here than attraction, because you are attracted to her and connected to her. Maybe talking about it like the medical condition it is, would help frame it differently for her.
I had erection problems starting in my early 20s after an injury and my erections changed drastically, so it was pretty clear cut in my situation and I've had to explain to women who might first want to interpret it as some sort of rejection but when I've explained it, it's been better. It can depend on the woman though. And it's unfortunate that so many have believed for so long that issues down there can only be mental or about attraction. There are many causes to issues there and that includes many possible physical causes.
Well, that stinks. First, let me say that a woman that makes hurtful comments about something you are struggling with is not really the nicest lady to be with. Maybe she is beautiful but her heart doesn't sound kind. Think about that . .. better to be with someone who has your feelings in mind and doesn't make careless comments. Perhaps it is performance anxiety which if you are intimidated, anxious, worried about failure might happen. You are erect until intercourse? Can you masturbate on your own? I'd talk to your doctor if you have impotence problems regularly How old are you. by the way?