I had a long-distance relationship partner who had this issue. He was fine with his wife, but he was unable to get and sustain an erection with me until after we had spent four weekends together.
He has low testosterone, but he was being treated for that when we met. I suspect that his treatment might have been somehow insufficient during our first few months together. He was basically functional after our fourth weekend together, but he never had what I'd call an active libido. I suspect he has some performance anxiety too and is just more comfortable with his wife.
I would recommend getting your testosterone levels checked. And even if your doctor says you're fine, find out your levels and do some research, since sometimes, what a lab says is within the normal range is not ideal for a particular individual. If you do think you are low, seeing a urologist would be the next step.
If the woman you really like, really cares about you too, she will hang in there while you figure this out. You sound like a very giving lover with a lot to offer. Good luck!