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There's a difference between feeling fat and being fat; remember that. At 5'4 and 120 lbs, you are at a great weight. I'm sorry you feel so large, but the body image issues should fade over time as long as you try and rectify them.
You could try telling yourself affirmations every day like "I love my body" and "I am beautiful". It's best to say these while looking in a mirror. You may laugh at first or not believe what you're saying, but after a while, it should help.
Try getting stronger. It makes me feel great about my body when I know it can do something amazing. Pick up running, lift heavy weights, join a sport...