Thanks for the thoughts, all. I'm sure that she and I will remain good friends even if I do need to break things off, in fact we talked about that when her poly status first came up.
I think what my heart is telling me is that I should not treat this as a serious relationship but as a fun thing that happens every so often. If she makes a move to commit things further, then I'll give it some thought, but in the meantime, if it's an LTR I want, I need to find someone who matches my personal ideal of what that means.
My squeamishness about this stuff likely stems from having been cheated on fairly heinously toward the end of a couple of LTRs. Both instances were handled awfully and in one case, with considerable malice. This was when I was younger and had that "I've found THE ONE" concept of love.
At the time, it killed me to think that the "love of my life" would do that to me, even though it was an artifact of both of these relationships being poor matches and far past their expiration dates.
So even now, the concept of my lover sharing herself with somebody else just makes me feel small, vulnerable, and unloved, which it clearly shouldn't. At the moment, this poly girl is treating me in a fabulously loving, caring, passionate manner, for the most part, and seems to be growing quite attached to me.
I suppose I should work this out with my therapist.