He obviously re-kindled his relationship with you because something is missing in his own relationship that he is getting from you. He must care about the other woman the fact they have been together for 20 years and the fact that he cant leave her.
I would ask him if he ever plans on ending it with her, if the answer is No, I would tell him its over, If the answer is "I dont know" I would say well you can blimin well stay with her, im out of it, and if he says yes he is planning on leaving, I would ask why the prolonging and set yourself a date, if he hasnt done it by then, I would have to say ta ta!!!
Your worth more than being a bit on the side!!!!!