Well it seems these are your choices:
1. Give up on sex. Probably the toughest, sex is one of the strongest instincts, and I doubt many Zity members would rush to recommend this. It would solve the problem but might sow seeds of resentment, although it was your own choice. If your own sex drive fades with age, this could become more palatable.
2. Secretly start sexual interaction with others ranging from non-contact chat etc to physical sex, e.g. paid-for sex or affairs etc. The former would probably be a lesser betrayal than the latter, but still is one. If she found out, and women are much more preceptive than many men realise, it would be hurtful and damaging to your relationship, especially if you’d chosen the physical options.
3. Explain the situation and try to negotiate one of the option 2 choices. She might accept this, but equally could feel she cannot refuse if now dependent on you through poor health etc. Not a nice predicament to put a loved one in; neither is yours.
There are no sweet choices here, but to paraphrase @JTMac, you’re a long time dead. I hope you find one that works for you both.