If you have decided to leave him, than just walk away, you don't have to feel guilty... you only feel guilty if you are still with him and he is kept in the dark... why let him have the chance to destroy your future? you see some guys will go telling about you and before you know it, everyone is speaking ill behind your back. People gossips esp in a case when the blow is too much for he/she to take, esp when they feel that they are the victim... they might just end up saying.. what have i done wrong in the relationship that my partner sleeps with another person? and you end up being condemn.. trust me.. people can say one thing at this time but when the real thing takes place.. you will be surprised their words differ from what they told you initially.. and i can also tell.. no one in the world is going to accept that.. we are humans not animals that after mating, forgotton about their mate.
you should just let go
1st scenario:you choose to keep quiet and you feel guilt tearing at you, one day you will be in the ayslum by thinking and fearing and feeling guity everyday
2nd scenarioyou tell him, you feel peace at heart that he knows the truth but another fear that he cannot accept comes in. ok assume he cld accept... he is going to treat you real diff and i can say that kind of life he is going to give you after he knows it = HELL. everyday, you will be metally emotionally abused by him.. you cant really confront him, he will use what you told him on you and I can tell since its your fault in the first place, you will be speechless upon his confrontation. he might go around spreading what you did to him.
3rd scenarioyou let go.. don tell him anything and move on.. give yourself time to be release from this guilt before you attempt another relationship, which can say... the guilt wont b long since the person isnt going to be him. you feel guilt because you cheated on him. Make aure you dont ever repeat the mistake again and you live in bliss.
if you try continue this relationship, your life might be in terror, nightmares... guys... their actions and words can go beyond your imagination esp if they aint the one at fault.
I have analyse it for you... the situations you might get into and sis yun had also advice you... many people had... you still eventually have to make the choice yoursef.. make a wise one, cos a wrong one = disaster. love yourself before others could love you.
cheers

may god bless you