Everybody handles backstabbing differently. No matter how old you are, some people can tell the person off while others are completely non-confrontational. Watermelon, am I right in assuming you are the non-confrontational type? If so, I commend you in asking for help as that is the most difficult first step. You have nothing to be ashamed of. But, as you know, as strangers, we can only speak from our own experiences and we do not know your situation specifically. Yes, I agree, life is too short. People should not have to spend their stress on worrying about stuff like that. So, all I can say is that there is no time like the present to become assertive. You don't have to be nasty or aggressive or violent. State the facts and get it over with. Like taking medicine, you know it will make you better it just tastes bad. If you feel like you are going to break down and cry - breathe deeply in through the nose and out through the mouth (and get away ASAP).
Confront your supervisor or the PITA and end the BS. Otherwise you will be caught in this mind game for a long time. You say your job is on the line so, really you have nothing to lose, however the gain will be self esteem and self respect - who knows you might not lose your job after all.