The difficulty you might be feeling in forgiving him might be your subconscious telling you something. Look at it this way: Let's name what he did. Say, he drove your car without permission, and scratched it. You might be angry, but you know he didn't do it maliciously. It's a big deal, but you can forgive easily because you recognize the intent wasn't to hurt you. If instead, he kisses your best friend, which could conceivably could be considered a smaller issue, you might have more difficulty forgiving him. His intention could be nothing but to hurt you. I don't know what happened, but if you are hanging on to it, maybe, just maybe you shouldn't forgive him.