1) Goverment gave itself right to decide what marriage is. Where such thing is written in Bible? Marriage is right and command given to us in Genesis. When did God transfer deciding about marriage from individuals/family to state?In one sense I couldn't care less because I'm a thousand percent fine with just her, easily. But being alone is HORRIBLE and I feel like I can't take it. I feel like if I married another woman, I'd have a hard time doing it through the realization that I'd slam the door on a marriage The big difference here is what it would mean for me to do that: an incredibly hard time getting her back if she changed her mind to a miraculous extent. She hates my theology as I stated, thinks I'm far from God, etc. She thinks that the laws of the land create the covenant which I think is nuts; I mean if the government can't decide the END of a marriage and God doesn't recognize divorces just because a government grants it, then why is the government in charge of deciding when a marriage begins?
Answer is never. You must teach your girl such things and have confidence that you are speaking truth because you know truth.
2) Woman hates idea of polygyny and she is still willing to stay with you? It must be true love. Otherwise, she would leave. In my experience she will get used to polygyny idea with time (years, not month) if you don't give her a inch. It will become just "you are such man with this not so strange idea".
3) You are suffering from oneitis. If your finance leaves you, you will never find such special girl. Otherwise, why would be so afraid of being alone? If she was just average girl for you, next average girl you could find next door. There is help for such cases, but I doubt this forum is right place for such discussion.