OK, here's an entirely different perspective from the ones you've been getting.
Polyfidelity.
Some people, and my lover is oneof them, are just not monogamous by nature. Society as we now it dictates monogamy as the only way, but its not. Some marriages are always bad because one or both of the people in them can't abide by the noton of monogamy.
22 years ago my lover got married. He made a vow and he tried to be monogamous. So finally after some counseling he and his wife sat down and decided he could see other women, but there would be rules. A long as he abided by the rules, it as OK. The rukes have changed over time, but always with agreement.
There are no illicit affairs, no cheating. It is all open and above board. For a few years he had multiple lovers. They all fit a specific mold and they followed the rules. I didn't fit the mold when he became my friend 16 yeatrs ago. He talked to his wife about me efore he uttered the first nonfriend word to me. She knew he was falling in love with me before I had a clue. The rule that had to change? I wasn't married.
Their rules....
Only married women.
Only when both other spouses agree and know each other.
There is a primary relationship - the marriage.
There is a budge to what can be spent on the other relationship.
Safe sex.
Permission from wife for overnight stays.
There were probably others, but they've lost their relevancy.
She happy, because he is happy and he is not off with strangers. She knows I am committed to their marriages success. I like this because I want the committment of a fulltime relationship, but I am way too busy to be married and far to protective of my space, and my autism makes me get overloaded by too much togetherness. It works for us. 3 people are very happy.
So would this work for you? I don't know. It's better than worrying about what she might or might not be doing. Can she be honest with you? Can you trust her to be honest? Can you continue i a relationship with her knowing she is with another? (I don't recommend multiple others - at a time.)
And BTW my son, I'm a single adoptive parent, has grown up with a mom who first dated, and now has a long term partner. He is so committed to myson that he is his back-up trustee on his financial accounts and medical proxies. He will live with him if I die and that makes the most sense, He has been an excellent co-parent in mnaty ways.
If you love her and don't want to lose her, see if this would work for you. You canhave the best of both worlds.