I agree that it is probably an affair. My h also told me that there is nobody else and will not be. However, I do not believe him. When strange behavior says otherwise, we have to look at the signs. He would not just be carrying viagra around in his jeans pocket. Please protect yourself. He's the one messing up, not you. Don't blame yourself. You are the one trying.
"Success is the sum of small efforts, repeated day in and day out." Robert Collier
"One's best success often comes after their greatest disappointments." Henry Ward Beecher