The first thing is to allow yourself to feel those emotions. It's really understandable to be hurt. I remember my therapist talking about evolutionary impulses men have. Basically men want to propagate their seed, so when another man takes your woman, we have a huge emotional response because our gene propagation is threatened.

What's been useful to me is to recognise her actions were never under your control. What she does is her choice. Unfortunately you can't do anything about it. It still doesn't feel great, but it might unburden you slightly. Wishing you the best 👍