I agree -- he TOTALLY shouldn't have believed her. But I am glad he did, because the scenes that follow and their mutual misery had me positively dying with angst. Clayton was definitely rash (not to mention his trust issues). And hearing that lie about Whitney would be enough to drive such a man (who was utterly besotted with her) to the brink of madness. I don't think he was like that in other realms of his life, but I like to think that because he DID love her so much, she brought out the best *and* worst in him, and that contributed to the extemity of his reaction.
But still I adore him.
And yes! The scenes that follow when he thinks she'll hate him (and ok, well, she does at the time) and never want to see him again!! Aaaah so sad! Maybe the brutal extent of his error changed something inside of him. Before he had wanted her, and was determined to have her at any cost, but after he did such a terrible thing to her, he sort of thought that she deserved better than him. *sniff* So he set her free to be with Paul, whom he thought she truly loved. Like Emily said, he had to love her to distraction to be willing to do that for--to do what he thought would make her happy even though it killed him to do so. OMG it is so sad.
Clayton was definitely an ass sometimes, but OMG, y'all, I love him. <3 <3 <3