How do I repair my metabolism?

I’m 14, 4 ft 11.5 in tall, and weigh 85.8 pounds. I don’t know if I’m under weight but I want to stay at this weight or get at least a little lower. I’ve been consuming only 120 calories a day for the past few weeks but will be forced to eat more. I don’t know how to maintain my weight if I have to eat more. Thanks.
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