Shipwrecked, I've been thinking about your post all day.
However, I wonder if there's another way to think about it. Lots of people find themselves alone / starting over / feeling that they made all the wrong choices at some point (perhaps at several points) in one's life. Later in life, it is surely even harder to go through that.
But it's not a problem specific to poly. One of my closest friends had to start over at age 53 last year, when her partner of 15 years left her suddenly. She had already been a cancer survivor and was dealing with residual health problems, chronic depression, and underemployment. Then she was left alone with 3 cats, a sick dog, and a lease that she could not renew on her own.
She and her partner were monogamous. Her partner left her for someone else.
Is monogamy the problem? Would my friend's partner have stayed with her if they had been poly and could have relationships with other people? Maybe. Would my friend be better off if she'd been poly? Maybe, assuming she had other partners who could be there for her when her primary partner abandoned her.
But...that's not the issue. Relationships can go horribly wrong whether mono or poly. It seems that you are questioning your choices to be poly, when you could just as easily have ended up in a long-term monogamous relationship that also ended badly and left you wondering if you'll be alone forever.
Maybe my friend would have been better off if she'd been solo (single by choice) for the last 15 years. She could have been building a life of self-sufficiency rather than expecting to be in a domestic partnership for life.
If she'd been happily solo, then suddenly lost her job of 15 years and realized she had no support system...she could still have ended up in the same place.
The point is, shit happens. Loving relationships can end horribly. A monogamous marriage can end in abandonment. One person's many poly relationships can all end at once. A solo person's house can burn down in a fire.
We tend to scrutinize poly more closely than other life choices (such as monogamy) because it's off the norm. There will always be a little voice in our heads saying, "This was a stupid thing to attempt" when a poly relationships fails. ESPECIALLY when more than one relationship fails at once.
Whereas, when a monogamous relationship fails, we don't automatically say, "Wow, the problem was monogamy, monogamy was a waste of my life, now I've ended up alone because I only loved one person." (Or, if we do say that, it's because we want to try poly instead!)
I sympathize with your pain. If you want to try a different approach than poly, by all means, do so. But poly itself may not be the problem.
And, personally, the fact that so many people are horrified by poly relationships and my dating pool is drastically shrunk...it kinda makes me think I'm onto something good (weeding out the chaff, as they say) and I'm going to stick with it
But I'm contrary like that.