A couple of thoughts....
- While it`s likely that she is trying for a upperhold on the situation, if you didn`t see any signs of poaching beforehand, I`m not sure that is the true reason. It might be as simple, as the fact, she likes you, but you aren`t the kind of person she feels she could handle living with.
- No, you can`t make her accept you. Your 'rights' to a boyfriend, are coming from the fact, nobody owns anyone. Your lack of rights regarding living there, come from the fact she OWNS her house.
Good luck to you. I hope she is willing to at least provide some answers in a straight-forward manner.