Welcome to the forums.
To me it sounds almost as if she's only OK with polyamory because she wants to make you happy. Sex is a very important part of any relationship and you may have a hard time satisfying a partner if you can't satisfy them sexually as well as mentally/emotionally.
If she can't figure out why she can't share you "completely" with someone else then polyamory may not be for her. It would be unfair to bring someone else into the relationship if you can't completely fulfil their needs as well. One of the biggest reasons polyamory fails (from my observations) is because a partner doesn't feel like their needs are met.
If she can work through that and share you completely, then you may have a fighting chance. As always though, practise safe sex.