How do you make a guy learn his lesson for cheating on you?

OK there is a long story and I won't be able to write it all, but bottom line is my boyfriend cheated on me. After not seeing each other or talking for a month he came to relieve that all he wanted was me and he made the biggest mistake ever. I want to take him back, but how can I trust him again? How do I know if he understand what he did was wrong? He says he does and that he doesn't deserve me, but how to I get him to understand that next time it happens, then I'm gone for good. this is our last chance.
How do you make a guy learn his lesson for cheating on you?
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