Summarize:

Express your sadness to your friend but explain that it’s something that you’re working to get through. Explain that you may be distant until you can fully get over your negative feelings.  You can say something like, “To be honest with you, it hurt my feelings when I found out you and Jen were dating. When I found out, it made me feel insecure and upset, but I’m working on feeling better about it.” If you suspected your friend and your ex were interested in each other during your relationship, it's okay to admit you suspicions to your friend. Hear out what they have to say before jumping to conclusions. It's okay to tell your friend how you feel. If you wish that you'd had a heads up about the relationship, it's okay to tell them that. If you feel angry or betrayed, make sure that your friend knows but make sure not to lose your cool. An honest conversation will make you feel better and give them a better understanding of where you stand.  If your friend doesn't take the conversation well, keep your cool by regulating your breathing and paying close attention to what they have to say.  You can say something like, "I just wish that you and Maron would have told me that you were thinking about dating so I could prepare myself. I'm still not over him." If conversations about your ex gives you negative emotions, it’s best to avoid conversations about them. Speak to your friend and ask them to avoid talking about their new relationship if possible. This may help you maintain the friendship and could make you feel better about the situation.  You can say something like, “Conversations about Jen make me feel awkward. Would you mind avoiding talking about her for right now?” If you don't get emotional when speaking about your ex, it might be a sign that you're over the relationship and can talk about them again. If you get over the past relationship, you can say something like "Remember when I said I didn't want to talk about John? I'm over it now, so if you want to talk about him, I don't mind." If you have to, take some time away from your friend so you can heal. Let your friend know that you still value your friendship, you just need a little time and distance to heal and process your feelings. If you find out that your friend is dating your ex, you might feel the urge to dish the dirt on your ex or sabotage the relationship. Ultimately, however, this could just end up damaging your relationship with your friend. Think carefully before sharing any information about your ex with your friend, such as details of your relationship or your ex’s previous dating history. Ask yourself whether sharing this information will accomplish anything positive.
Have a conversation with your friend about your feelings. Be honest with your friend. Ask your friend to avoid talking about the relationship around you. Resist the temptation to badmouth your ex.