My initial impression reading your story is that this is about what happens to relationships that aren't sustainable long-term when NRE fades. Which happens at different times for different people. In the initial rush of new romance everything seems perfect...then reality steps in.
He experiences this (i.e. realizes what he is really searching for/looking for/wants) and realizes that he doesn't want to share (even though he knew at the onset that this was a condition of the relationship). For you this takes the form of feeling like you fall in love too easily and then your love seems to fade over time.
If you take poly out of the picture the same thing may have happened but it would have taken a different form (i.e. you would have been with him for a few years and then moved on in a "serial monogamy" type of fashion as you gradually lost interest). If you learn to recognize your pattern for what it is you may be able to soften the blow for yourself and others in the future.
Now, I may be completely off-base! Personally my tendencies run in the other direction - my brain "refuses" to acknowledge my feelings for someone until well after my NRE has run its course and I realize how much I turn to someone for support in a "daily grind" scenario - until then, apparently, I tell myself that it is all about sexual compatability with a good friend and the excitement of MrNewBrightShiney.
Another point to consider (again I may be completely off-base for you personally) - just because a relationship is not "long-term" does not mean it was not worthwhile. Not every relationship has to evolve into a "marriage-or-its-equivalent" to be successful - this is a goal-oriented approach in my opinion, sometimes they journey itself is more important. Some relationships just run their course and both people are better off having been in them even if they are no longer together.
My two cents (which in these inflationary times, is practically nothing).
Me: poly bi female, in an "open-but-not-looking" Vee-plus with -
MrS: hetero polyflexible male, live-in husband (together 21+ yrs)
Dude: hetero poly male, live-in boyfriend (together 3+ yrs) and MrS's best friend
Lotus: poly bi female, "it's complicated" relationships with Dude/JaneQ/MrS
TT: poly bi male, married to Lotus, FB with JaneQ
VV and MsJ: bi-women with male primaries, LTR LDR FWBs to JaneQ
My poly blogs on this site:
The Journey of JaneQSmythe
The Notebook of JaneQSmythe