What do I do?

I just caugt my b/f cheating on...but when I asked him about it ...he said it was his cousin(which i don't believe) I just wanna know whether i should give him a nother chance b/c me and him have been through some things for real, and i really miss him...but this is not the first time he has done this, he has done this 1 time before....just let me know ya feeling and help me decide what to do!!!!!