Have you talked to you husband about how open he would be to you having sexual gratification outside of the marriage? Honestly, the place he's in (elderly, impotent, introverted, sick) he has to be very well aware that his current state is absolutely tormenting, and stifling, his younger wife. He might be more open to the idea of you finding romantic and sexual fulfillment outside of the marriage, which isn't really even much of a marriage to begin with to be honest, as long as you remain his care giver and companion.
I actually think it would be quite cruel of him to expect fidelity at this point, seeing as you're staying mainly for the sake of taking care of him. Your "husband" doesn't treat you like a wife, he treats you like a friendly nurse. It's unfortunate that his life, and your life, and the choices you both made, have landed you here, but something has got to give.