While it’s true that your body’s metabolism will slow down the longer you diet, it’s not necessarily true metabolic damage. After all, both your body weight and food intake will be lowered, so it makes sense that your metabolism will slow down. Unless you have a history of eating disorders or have dieted down to dangerously low levels multiple times, you probably don’t have real metabolic damage—you just need to lower your calories, as your body has adapted.
This could be due to several reasons. Most likely, your body’s metabolism has simply adjusted, and because you now weigh less, you don’t need as much food, so weight loss will slow down. You may also see a slight decrease in the production of hormones that regulate fat loss, such as the thyroid hormones. In more extreme cases of prolonged, severe caloric restriction, it’s possible that your body will actually fight weight loss because it thinks that it’s not getting enough food, so it wants to hang on to the energy stores it has, but this is rare, and most of the time it’s normal metabolic adaptation, not a health issue.