At some point you need to be honest about what you are going through.. If you cant do a face to face talk right now , why not type up a letter to them filling them in on your mood swings and how unhappy you are. explain in the letter how you fear what there reaction will be and you are just "Needing" to find out what is wrong with you.

Chances are after the shock wears off your parents will probably feel bad for not "seeing" something was wrong.