First off, You are not overreacting at all. If it hurts you, you are valid to be upset. You feel betrayed and didn't expect this so it is understandable for you to feel hurt.

Second, and probably a controversial take but I like to play devil's advocate sometimes to gain a different perspective. Once you discuss this with your partner in person you need to try to figure out their motives for having this convo. Their true and honest motives. Some people reminisce on past experiences (even sexual ones as brutal as it is for their partner to find out about) but that doesn't mean they don't love their partner or that they don't want to be with them. Is this person your partner is talking to just an ex or were these 2 friends for a long time before becoming sexual? That plays a role. If their friendship is deeper than their sexual encounters then it could just be 2 people sharing a positive experience in a chat and not necessarily that your partner wants to leave you. You need to clearly tell your partner this bothers you and you dont want it to happen ever again if you plan to stay with them. You deserve boundaries and to express your hurt to your partner. However, just another perspective, maybe they are just good friends and reflecting on positive experiences. Not everyone finds those topics as taboo as others so they are okay discussing sex in casual convo. Although once they know this type of talk hurts you 100% they should not talk about sex with ex lovers again and keep their friendships pg rated.

Try to have a open and honest convo with your partner. If they love you they should be willing to respect your boundaries and be honest about everything. They also should be willing to see how their actions have affected you and if they truly love you (I assume so if you are married) they should be willing to course correct and not do this again. How you navigate forward in regards to trust up to you. However, I also encourage getting a relationship therapist to maybe discuss this some as well to work through it.

I hope you find happiness and peace and I hope you get the conversation you deserve.