This is a tough place to be, and a lot of the thoughts that you are having are not unique to you - I have heard it many times before.
Have you been able to identify to yourself what concrete aspects of the relationship you are needing in order to make the relationship fulfilled? If so, have you communicated that to them? Knowing exactly what you want is really important when it comes to poly, because a lot of the "standard stuff" that is assumed with a monogamous relationship becomes non-standard, package deals can be broken up and the parts taken separately.
Now it could very well be that what you are looking for cannot be provided by a guy in a live-in relationship, or specifically, not HIS relationship. If that is the case, then you have a choice to make - become poly, and, while keeping with him, look for others who can give you what you want, or stay monogamous, and leave him and find someone else that can give you what you want.
Are you in touch with her, or just him? Have the three of you sat down and actually talked about the boundaries, needs, wants, etc, or the relationship configuration you are in? That can be invaluable in sorting through some of this - a prerequisite for it, though, is that each has a pretty good idea of what they each need and want and what their boundaries are.
Please check out The Birdcage - an open, friendly Polyamory forum for all parts of New York State
"Listen, or your tongue will make you deaf." - Native American Proverb