How to not let him manipulate me again?

I've been dating this guy for a couple of months, and I love him. But he treats me really badly. But every time I take the decision to end it, he becomes so nice and I cave in. However this time I was really determined, and I told him it was over. We work together, so every time we see each other, he gives me the saddest eyes, he looks miserable. It breaks my heart to see him like that but I'm not even sure if it's real or just an act. I still love him but I can't continue like that. So what should I do to not cave in again?
How to not let him manipulate me again?
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