The way I understand it, Tomoe has forgotten his love. When Makage cured his curse, it was by making him forget and I think she was part of that loss. In the episode, he said something about forgetting little by little over time.
Also, the eye was a cure-all. I don't think it granted immortality. He's been upset for hundreds of years, so likely she just died of old age. It's probably why he's so opposed to Yokai and humans dating.