The relationship is already over because you don't trust him and he shouldn't trust you because you're a sneaky person who looks through his things actively seeking to find clues that will support your lack of trust in him. The relationship was over when he "did this" (you didn't go into detail about how he's cheated on your by flirting with other girls) before. That's the thing though. . . if you stick around in bad relationships where one person is definitely not interested in commitment, but you expect something they refuse to give you, then you're going to not trust them. Don't be the girl who is too weak to end a relationship and who turns into a privacy-destroying monster in an attempt to force him into monogamy. That's a bad look and a bad pattern of behavior to learn.