If your GF is truly Poly, you will not make her happy by forcing her to be in a monogamous relationship. Forcing a non-monogamous person to lead a monogamous lifestyle is incredibly insensitive and selfish. It is like putting a salt water fish in fresh water. It is not an elephant, it is who she is. If she had to go find someone else, it is because you weren't giving her what she needs. If she felt like she had to find what she needs elsewhere, that is not her fault. If you truly loved your partner, you would do what is best for her or let her go. Love is when you help your partner be who they are and become who they want to be. Are you willing to do that? Remember, you can be in love with someone who is not your best choice as a life partner. It sounds to me like this might be the case here.
55 y.o. unmarried straight male w/straight GF wishing to meet women for dating and relationships. Also interested in just meeting people for fun and friendships. Blog and email buddies welcome!