If you want a middle ground, you need to know what you are fine with and what you aren't. For instance, would some sexual activities be fine but not others? Would some people be acceptable partners but some others? Are there things you would want to remain exclusive to your relationship and that you don't want her to have with others?
We can't know these for you, but if you can make your own list and run it by her, then a compromise might be an option.
As far as understanding how polyamory works, I think hanging out around here and reading people's posts is probably the best way to learn more about it