I struggle with this a lot too
I feel you! And this question has been in my mind for awhile.
For me, personally, as a near middle age woman has been:
1. Married men wanting to cheat.
2. Youngsters who want to eventually get married.
3. Poly people already in a relationship.
4. Older men who are single (but there is a reason they are single! They tend to be quirky.)
5. Poly single people.
So, the only possible combinations for lasting relationship SEEM to bee someone who is young, single, and open to poly coming into our family. Or an older man who has been there done that and wants a lover wihtout a partner or an already married poly guy. (I actually only met one person like this).
Many of these combination seem hard to obtain. I think organically people want more of their lovers.
Yes. people break up and die. But there is a huge difference say with frieds - you expect one day they will die or break up...but the relationship is solitified in a different way that creates less anxiety. I guess I would ask (both OP and others) what the combinations might be?