Im having problems with my Christmas Cactus...?

I would get it out of the pot and check the roots. My guess is they are rotted from overwatering. Start over: Get new potting soil and repot the what appears to still be alive. Be sure to bury the segments that are alive and well down into the soil. New roots will come out between segments. When you take it out of the pot, carefully pull way the rotten parts until you find roots that may still have a chance. Keep the soil damp, but not wet. Remember, this plant is a succulent and doesn't need a lot of water. People tend to keep these for years and years and sometimes they get very pot bound and will go down hill because there isn't any soil left to hold moisture ....either of these things could be your problem...only you will know when you unpot it.