@Tattiana ,
Relationships are built on trust. And right now you are having trouble trusting him.
A few things. He will have to earn back your trust. And you might want to work out some of these feelings with a counselor, e. g. he told you, it was apparently a one-time thing, if you want to stay together, you are going to need to learn to move on.
But I think this is a symptom of being together for 8 years, being each other's sole relationship and ....? People get curious and they can sometimes act out. I am not saying it is right or fun or lovely but it is understandable.
You may want to have a conversation, instead, about where things are going. Marriage? Living together? What's around the corner for the two of you?