You are in a very difficult & heartbreaking place. Please consider, however, that your H is likely depressed. Men who find their youthful virility disappearing generally do sink into a depression, and depression makes it almost impossible to to imagine that things could change, that a situation isn't hopeless and irreversible, or to force oneself to act when action is necessary.
Depression also makes it impossible to appreciate another person's point of view--don't imagine that he doesn't care about you, or is cruelly ignoring you. A depressed person can't deal with his own misery, let alone get far enough outside his head to comprehend his spouse's misery.
I know you think you would go to a doctor as soon as you know you had a sexual problem, and let's hope you would. However, if you were depressed, it's very unlikely you'd have the energy or perspective to do so.
Sorry, I don't have any answers for what you could do, other than talk to your doctor about the situation. Given that your self-confidence is slipping and your vulnerability is increasing, and there is a lurking danger that you will grow to resent the "SOB," it would probably be useful to get some counselling for yourself, so that you feel supported. Dealing with a depressed person who is now inhabiting the body of someone you loved is an incredibly difficult and painful job.
