You need to tell your boyfriend. We all make mistakes but you sleeping with him not only once but twice shows how little you actually care about his feelings. With these sort of situations we need to look from both sides. Imagine how you would have felt if he slept with his ex while you were away and then didn’t tell you. He may understand and be ok because he was away, but he deserves to know to be able to come to that conclusion. Also, relationships run on trust, you’ve already made the mistake so now it’s your chance to be honest and get it off your chest. Also, the fact that you have gone back to this guy for sex shows that your bf isn’t your one and only, especially if you want to do it again.