Re: Sex failure after 20 years
OP, sorry to learn of your dilemma, and welcome to TAM.
I share your problem. My wife isn't doing much in bed (too much work), and her behavior isn't conducive to getting (or keeping) an erection. I say this as a guy in his mid-30's, so the problem is quite bad.
I've been offended by my wife's behavior, and have flat out refused sex on her terms (no favors, no foreplay, quickies-only, etc). This can be a good short-term strategy that might lead to your wife putting in more effort and changing her ways in the bedroom. It's worth a try.
In the long-run, if your wife won't change her attitude about sex and you keep avoiding her, she will feel rejected. Be warned that this can create even more problems in your marriage, versus solving anything. It is at this point when avoidance nets you an emotional loss, and you should give her sex. If the woman wants vanilla sex, then give her vanilla sex. It will help your marriage even if not your own problems, and that's worth something.
When I'm getting busy with the wife and the complete lack of her foreplay or sexiness challenges my erections, I'll engage in enough foreplay on her to get an erection. But then, I'll stop foreplay in order to strike while the iron is hot. For you, if it's hard (no pun) to keep an erection with too much [one-sided] foreplay on your wife, stop the foreplay and get down to business. She'll be frustrated with intimacy (as you've described) if you deliver foreplay but not actual sex, so it's perhaps best to have sex when you can? Leave the foreplay for after sex, if she's still up for it.
Sorry I can't be of more help. Good luck!