sorry, but neither of those is correct. "You are being sad, when you should be glad'', "you are being sad, instead of being glad'. "Sorrowing', while doubtless a technically correct word, isn't really used, expect perhaps in poetry. We would more usually say 'grieving'. "Saddening' also isn't really something we would ever say - I guess technically it means 'growing sad', 'becoming sadder', or perhaps even 'causing to become sad' - but whatever its theoretical possible meaning, it simply isn't used as far as I know.