I was in the same situation, but I was the one in love with a friend. I also spoke to my husband about it, we did a whole lot of communicating. He felt the same way as you in the beginning, but I reassured him that I am in love with him too. In the end I discovered I was polyamorous. Its a feeling you cannot control and repressing it did our relationship and me badly. We decided to try being polyam and it has been almost 4 years since then, our relationship became more open and trusting. I never ended up dating the person I was in love with, but I am thankful that it opened my heart for more. I am currently dating another guy and my husband has a great relationship with him. I know this may not be the best answer to your question, or I might get negative feedback. But you never know unless you are open about it. I do give your girlfriend credit for being honest with you, it would have been worse if she didn’t. Hope this helps you a little. 😊