I can totally relate to these emotions. I have posed the question to a few people who are part of a couple, looking for an outside relationship: what do you have to offer this other person? How do you see it working for them? The question has been brushed off with platitudes, and yet, it strikes me that there are very few secondary singles, or mono secondaries, and not even that many secondaries, posting here. I see what they post elsewhere on forums and e-mail lists more geared toward the mono secondary, and what I see is a lot of pain and uncertainty for the person outside the core couple. Yes, I know there are exceptions. That doesn't change the amount of pain that does exist.
For reasons related to what you discuss in your letter--for reasons including the fact that you don't feel you can say these things to your girlfriend, just as I have found that when I raise similar issues with my boyfriend, he tells me I'm imagining things or gets angry--I would never do this again.