Its been 6 weeks since I split with my cheating ex. I heard that he has moved on. Should I do the same ?

Its been 6 weeks since I split from my cheating ex. One of my friends who got on really well with him, is still in contact with him. I am not happy about this and she is aware of it because I asked her 3 times until she finally admitted it. I have recently met someone new, he's cool not my usual type at all, but he's sweet. I don't want my ex back, but now feel resentful that he has moved on. I am considering letting this new one go because I feel that Im on a rebound. Why am I so angry about my ex ?