How to confront a cheater in relationship?

I'm currently in relationship with a guy for 4 years and he slowly started to ignore my messages and calls and even meet up. Lately i found him in touch with his ex gf. when i asked him, he denied and put the blame on me back saying that he is not. 

I'm not sure what m i suppose to do next. I'm worried and getting depressed each day. My works getting loaded and i cant concentrate in my work. how to confront a cheater in relationship? what should i do? to save the relationship or let him go? 

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