First thing - I think you had no problem expressing your fears here in writing. Why don't you just print this out, give it to her, and sit down and get into it. The result might surprise you
Secondly I'm going to offer up something that I feel (and suspect most here will agree with?) may be a fact of life (human) that you may not have become aware of yet. And I think this is one of the "realizations" that are driving things like polyamory forward.
Despite the obvious exceptions (and those exist in anything), humans are multifaceted beings. What we "need" to be happy, secure, fulfilled is not likely to ever come from a single source. When it comes to relationships there's this theory out there that if we can only meet this theoretical Mr or Ms Right then ALL our needs will be fulfilled. I reality - at least as I've seen in my lifetime & experiences - this is wholly a fantasy and unrealistic. Many attempts I've seen have at BEST resulted in people being severely restricted and never fulfilling the potential they were born with.
The "dream", in poly and other areas of life, is about finding a better way. A way that we can be "all that we can be", WITH support & love from all those around us that want that same dream for us.
Does that make any sense ?
And to address some of your more specific concerns about "cheating" etc I can only say that I think it absolutely unacceptable. Have others done it and even made it work for them (turning a blind eye, don't ask-don't tell etc) - yes. It's happened. But my guess is that if all of us here were gambling people - it's a bet we'd never take. Because if we 'lose', that loss is substantial. And it's not limited to the 2 (or 3) individuals involved but ripples out to endless others - society in general. And from a purely philosophical standpoint - it just doesn't fly. Period. Honesty is fundamental.
I guess I'd suggest you just spend some time soul searching and trying to really connect to what it's like to be truly human and all that implies.
You (like many others) may just not be able to get your brain around the concept that we're capable of loving multiple people without limitations. If you can never get acceptance of that for yourself then you will never be able to be genuine and in a relationship with someone else who see's life in a dramatically different manner.
Can we all get along ? Absolutely. Can we care about these "others" - absolutely. But can we truly "bond" with them at a level necessary for a romantic (I hate that term) relationship - highly unlikely.
Take some time alone. Think. Study. Be "true" to yourself. Lacking that you can't be "true" anyone else.