I think your best bet for finding a longer term secondary is to be explicit in your questions the way GalaGirl suggested. Or look for a secondary who already has a primary and therefore is less likely to miss the aspects of a relationship that a primary can provide.
I wonder if some of your secondaries who broke it off saw any other options.
I am a secondary-only. It's not easy. I, too, wanted to end my relationship with my SO because it was very painful to want/need some of the things that usually go with monogamy or a primary partner (moving in, sharing day to day life, having a foreseeable future, kids, etc.) and not have those options available in my loving relationship.
I recognized that my yearning for something more was not tied directly to my SO. Ending my relationship with my SO wouldn't change my wants/needs. Granted, having my SO so close and yet my needs be just as far out of reach was and is tough.
I work on these feelings, much the way I see others work on jealousy. They're not constant. Anymore, I'm doing great, with occasional pangs. No relationship is perfect.
My SO helps, he can be a tireless ear. The passage of time helps. The one year mark (about where my past monogamous relationships usually either got serious or ended) was particularly tough. I posted about it here a while back. Iím happier now. I got through it. A person can get used to just about anything.
I've even read how those with primaries can go through the same thing, wishing they could have more with a secondary when that's not always an option. We deal.
I toss this out there to give your future secondaries another option and a smidgen of hope, if they're up for it. Best of luck.