should I tell him I know

Hi everyone I went on our laptop the other day to sort through some emails, and it was logged into my boyfriend's email account. I noticed emails from zooke an online dateing site! so I clicked on one of the emails and he's set an account up. I feel upset and disappointed that he no longer wants me☹☹. We have been together for 10 years. what should I do I don't want to loose him xx.