Stepping outside a committed relationship is often a sign that the "stepper" has a problem, not the "steppee."
The simple reason is an inescapable feeling of unfulfillment and dissatisfaction with life, usually unconsciously with oneself. Namely the Existential problem.
The more troubling reason may be a past trauma or traumas. This sometimes results in compulsive repetitive behaviors, like gambling, shopping,
or sex. The steppee brings the trouble to the outside relationship, as may his/her outside partner.
They may be in a state of excitation, but not in love or happy. There is guilt!
It's all devastating and destructive.
In short, even if this is your problem, it may not be your fault. Be strong. One day at a time. Do what is necessary and right.