"What I am, is bad at monogamy....
I have cheated on monogamous relationships in the past. Sometimes after 1 year, 2 years, or 7 years. Sometimes this was because the relationship really needed to end, and sometimes not."
me too, Strixish.
for what yet another opinion/perspective is worth, MoBetter: i applaud the fact that the question you're (both) asking is "do we really want to be together?" and also that you recognize and acknowledge that neither of you wants yourselves or the other to feel suppressed or censored for simply being who you are - which may mean that you need to split in order to be fully yourselves, if you are mostly mono and she is truly poly (which is still up in the air).
what if you both agreed on a split for, say, a month or two? could you agree to spend this time apart, with no ill will or prejudgments, maybe decide to keep in contact (whether by phone/email or in person, and maybe on a schedule) simply as friends with the intention of giving yourselves enough room to breathe and think clearly (especially her)? then she might be able to distinguish between "i'm poly, and it's just who i am; i need more than one person in my life" and "i'm mono because i want to be" vs "i'm mono (unspoken: because i really need you in my life and that's the way it has to be if we're going to be together)."
just a path i might take, if i found myself and a partner in that situation. for what it's worth.
best of luck to you both!