I Get The Movie!!!!!
Someone may have written this already but here it goes.
Sean WAS her husband but it was his soul that was reincarnated. Remember that a soul cannot be evil it can only be good/love. So his soul (which was really him) loved Anna. His ego in his past life was what caused him to cheat on Anna with Clara. Therefore he would have no memory of this because your ego is in your mind and it didn't transfer to him in his new body, only his soul which is good. When Clara told him about him cheating on Anna it put doubt into him about ever really being Sean. He couldn't tell Anna why he changed his mind because he loved her so much that he didn't want her to ever find out what the past Sean did. Does this make sense? So it's the ultimate show of love by holding back the past.