breaking up

Ok I know I need to break up with my boyfriend... I'm just so attached to him but after him cheating on me, I know I need to leave him. 
I really need some advice on how to get my heart to let go and what to say to him and when to tell him I want to break up. Ugh I just need help!! I've never dumped someone before and we've been together for a year and a half. I just don't know how to break up... lol