Perhaps her soul is in torment. She herself *needs* peace of mind about her murder, because it *was* traumatic. She's in-between. Not part of the after-life, and not part of real life. It could be tortorous for her to love Clark, for him to see her, but not be able to physically touch him, make love, etc. Perhaps everyone around Clark is thinking he is crazy because he is interacting with an invisible person. She could be holding him back and creating a negative social impact on him.
Perhaps her 'moving on' will not seperate *her* from Clark, but it will free her spiritual pain she is in. Lois is a 'ghost' of herself. She may not be the entire Lois that Clark knows and loves. She is an emotionally tormented version of herself.
Just spinning off ideas, seeing if any of those help. I can throw some more out if you need. Ghosts are something I've read up a lot on.
