How can I get the strength to stop eating so much?

ok I'm on a diet well I'm meant to be because I really want to slim down but I'm ok during the day I eat healthily and don't be hungry but then at night I get so hungry and eat everything! I weigh 138pounds I want to lose 20 pounds. can anyone please tell me how I can get control over my eating? and how long will it take to drop 20 pounds?
How can I get the strength to stop eating so much?
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