How to break up with my boyfriend in a way that he will never forget?

I have been with him for about a year and four months. I cared about him, but I don’t love him. At one point I thought I did.
Anyway, to the point — I caught him texting, instant messaging, and even saw him on pictures with other girls posing in a not so friendly way, rather romantic. It hasn’t happened once, it has happened many times. I even broke up with him once for it. But we got back together for some stupid reason.

Now, I caught him again, but instead of just exploding in anger, I kept cool. He doesn’t know that I know he’s cheating. Little does he know what’s expecting.
I have pictures and even took a screen shot of one of the conversations. I’m going to throw that to his face.
I want some mean ideas on how I should break up with him, and mean things to say to him.
I don’t want him to forget this ever.

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