Whenever I try, it seems so unreal and forced. Does anyone have any tips?
As far as making it seem real or not, if you clarify to your readers WHY the character is feeling so dark it can make it feel less forced. Just have a play around with the words a bit =]. Also, if you have a friend or beta to go through your work do so. Things often seem worse in your own eyes so it is better to have someone else look at it.
Show it instead of saying. Tell what the characters have been through, in terms that vividly convey (without saying) how hurtful or frightening those experiences were. Also describe the responses or behaviors of the character that would let the people around them know, just by observing, how unhappy the characters are.