Leave him or keep him

So me and my bf have been together for 8 years since I was 14. He cheated on me when I was 15 and he was 14, so I cheated on him too a couple times. But since then we've been really commited. Until I found out last week that he's slept with my sister and my friend. I've come up with a few revenge ideas, which should I do? 

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