Thanks, all, for your helpful comments.
Ariakas and ray, I absolutely agree that a poly relationship would involve at least as much commitment as a mono one, and for that reason wouldn't automatically solve the issues I've had in previous ones. I fear there is a part of me that wants to see polyamory as a "quick fix" to the worries I experience, but I don't fully trust that part of myself.
Tonberry, thank you for your thoughtful reply -- it's given me a lot to think about. I do think the feelings I have are partially due to "not being allowed" to pursue other people... but I don't know if that's all of it. In my life in general (i.e., non-romantically as well as romantically), I often struggle with indecisiveness, worry, and self-doubt. So this makes me think that that might be a more core issue, rather than it simply being a matter of me not being "wired for monogamy." But part of me does feel that I would be able to commit myself to a relationship more easily, and worry less, if I knew I was still open to have experiences with other people I might meet. I don't think I would even plan on actively pursuing other partners -- I think just the sense of that possibility not being closed off would be helpful to me. I'm not sure what that means, if anything. Part of me thinks it just means I'm selfish, not polyamorous.