Your situation is very common. People with a cool/dismissive type of attachment have a difficult time understanding their own emotions or what motivates their behavior.
The research shows that people with a cool style of attachment often didn’t have their needs met early in life (or in an important adult relationship) and learned not to trust others, which leads to a lot of distress. A common way of coping with such stress is to ignore their emotions. The research shows that people with a cool style of attachment have emotional reactions (like everyone else), however, they’ve just learned to ignore their feelings – it’s easier than dealing with their underlying pain.
And because they have a difficult time trusting partners, they often act in ways to put themselves ahead of the relationship – like cheating. It makes them feel safe. Of course, they don’t understand this.
I’m not sure that this is what’s going on in your relationship, but this is a common pattern with people who have a cool style of attachment.
Working with a counselor or therapist is your best course of action. A good counselor should be able to help your partner identify and work through the painful experiences that led him to discount or deny his feelings. Ideally, a therapist can help people with a cool style of attachment learn that it’s healthy to recognize one’s emotions and learn to trust partners to be loving and supportive.
It’s also important for you to take care of yourself. The feelings you’re experiencing – increased insecurity, frustration, and uncertainty are normal in such situations. If you can, find ways to practice self-care. Take time to treat yourself with compassion and kindness. Understand that you’re going through a lot – be sure to remind yourself and that you’re a kind and loving person and the situation you’re going through is temporary. Although it feels like you’re stuck, give your partner sometime to work through this process with a counselor or therapist. Keep in mind that you’re doing everything you can to resolve the issue and take pride in the fact that you’re handling what happened in the best way possible.
Hope that helps.