"Meanwhile, the two women I am interested in continue to pursue other, more traditional relationships (which I am fine with). However, the relationships tend to leave the women unhappy and unfulfilled (which I am not fine with)."
Not your issue. Not saying that to be mean, but really, you're not parent, big brother, or even husband guiding your wife into how to have a more fulfilling relationship with her poly partner. Sounds like you're just trying to play rescuer.
"Of course, I'm just self-aware enough to realize that I might be prejudging those relationships out of jealousy. I don't really interfere, I have met and like the guys all right, but some selfish, jealous part of me believes that no man they choose could ever love them like I do."
If you are not in the running, you don't get to choose the contestants.
"Further, the impression I get from the first woman is that while she doesn't think being with my wife and I is possible, she judges every man she gets involved with against my standards. This has ruined at least one of her relationships."
Okay, again, not trying to be mean, but this smacks of presumption on your part.
"If I love her, and I do, can I let her keep doing this?"
Why do you think you control her? She may see something in you she likes, and may be seeking it in someone else. So what?
"Do I need to "withdraw the invitation" so to speak, or distance myself from her somehow? If she can't be with me, I can't stand to make her unhappy. While every bone in my body wants to cling on to the desperate hope that she will have an epiphany and we can be together one day, the other part of me doesn't want to ruin what chance at happiness she might have without me."
If you offered, and she declined, it seems to me there is no reason to withdraw it. And frankly, it is not in your power to ruin her happiness.
"I guess the only advice I'm asking for is how to deal with the emotional strain of realizing that you care for someone very deeply, and they care for you, but the life you envision between you just can't happen, or doesn't seem to want to happen. It's all very frustrating,"
Agreed. That frustration kicks in for me when I have a high interest in "making it work," and the other person doesn't, for whatever
"and in the end I simply don't know whether I'm just deluding myself into believing that a poly lifestyle is tenable in my situation, or if I've simply fallen for the wrong women, or if I'm trying to hold too tightly on to the women I love because our relationships will never be what I want them to be, or what. I just don't know how to deal with never. It makes me feel young, and it makes me feel stupid, and I don't care for it."
Well... sometimes the writing is on the wall, and we don't want to read it. Sometimes we hold onto things because people send us mixed messages. We're afraid to let go because there's a 10% chance it could work, even though we know damn well, in our heart of hearts, the 90% is what we're really dealing with.
You may as well let go. You can't control it anyway. You don't have to be an ass about it. Letting go of expectations is just that. Nothing more, nothing less.
Sigh... I should have taken my own advice two days ago