My stomach hurts

Yesterday was a bad day for me and ken
He lied to me, went behind my back, and tried to cover it up.
And to top it off, it was with a 'friend' of mine
I can't believe he did this to me.
After all the times he claimed he 'loved' me and promised he would always be there for me
and he would never do anything to hurt me, he goes and does this. How could i be so blind?
All guys are the same. Now i have major stress cramps in my stomach, and i feel like shit.
I called my friend Katie, and i told her that me and ken were going to try and work things out, but i wasn't sure how well that would go. She gave me some good advice, and said that he's not worth my tears and my pain, and that i should break up with him, because if i forgave him for this now, he will probably do it again. I really don't know what to do..
My sister was amazed that i haven't broken up with him yet. She was all "Wow, you must really love him to be able to forgive him". And i do love him, just i view him soo differently now. It's hard. And he says he wants to prove it to me that he will never do it again, or cause me any pain ever again, and he says he will always love me. I don't know what to believe, who to believe, or what i should do..