It happens. Blood flow changes are probably responsible and it happens as we age. Have you talked to your doctor about it?
It’s your diet. Poor blood flow is associated with the Arteries being clogged. Drop all junk food, eat grass feed meat, veggies and cardio to improve heart. Look up unclog arteries naturally on YouTube as well.
At age 66 this can happen and, in fact is not all that unusual. Your hormone levels have declined - it's part of aging. At age 65 approximately half of men are no longer interested in sex. Of course you only hear about, or see on porn the other half because that's what's generally interesting. Viagra gives most men erections but you have to be truly sexually stimulated to get one. It's not automatic.
The 6" down to 4 1/2" thing? Are you obese? Or have you gained a lot of weight? That's the most common reason for decreased penile length. Much of it gets buried in the fat pad at the base of the penis.
At age 66 preventative stuff like a special diet is unlikely to undo what took 40 years to happen. It is what it is. Life's short, and at that age, life could be a lot shorter than you think. Don't punish yourself. Enjoy the years you have left.