I, personally, have never held a secret so long that it had any effect on my composure or anything of the likes. I normally stay away from things that would make me feel bad because I know all too well how easily I feel guilty. I don't really have much to say about keeping a bad secret because I really never get involved in anything that would be considered bad.
Whenever I feel guilty, though, I really can't think. I'm always wide awake, thinking of what to do to make it better. Sometimes I feel like I'm even going to throw up.
Its not really that often that I'm in those situations though.