Very interesting question! Really got my mind turning ...
The way I see it playing out (and this is just in my mind, enter at your own risk LOL) is that like you said, Clark would *try* to tell Lois but would fail ... perhaps he does not want to risk telling her and putting her through the pain in case he doesn't return, or perhaps he figures the shock of learning he is Superman and that, oh by the way, he is leaving Earth to go off and fight a bunch of bad guys on a space rock, would be too much shock and pain to inflict on her. So, doing the "noble" thing (in Clark's mind anyway LOL) ... he leaves without saying goodbye. Naturally the world knows Superman is leaving, but Clark just seems to disappear. Clark sends a goodbye note to Lois, that is delivered to her after he is gone. He doesn't reveal that he is Superman, however he simply tells her that he decided to leave Metropolis because they "haven't exactly been getting along lately" (think
Whine, Whine, Whine), tells her that he loves her and she should get on with her life. After some time Lois puts all the pieces together and realizes that CK=S ...
Okay, maybe I got a bit carried away!