Not all of her past relationships have been a open relationship so I wonder what is it about me that she needs someone else in her life.
I'm in an open relationship not because there is anything wrong with my gf, but because I like variety and adventure. My gf satisfies me many ways: emotionally, intellectually, and sexually. There is nothing I would change about her.
Your gf may feel the same way. Why not ask her what attracts her to another person? Do not assume you have a problem. Some people simply crave variety. Everyone has something special about them, but every person is special in a different way. Me and my gf appreciate each person's "specialness" as something only they can bring.