Well there is two likely possibilities. Either you're depressed. Or you've chosen the wrong field/subset of a field. (Or even both maybe) So try psychotherapy/

Well there is two likely possibilities. Either you're depressed. Or you've chosen the wrong field/subset of a field. (Or even both maybe) So try psychotherapy/seeing a psychiatrist. And try taking a course on another subject that might interest you. If you find out that what you're studying is not interesting you anymore there is no shame in changing, be careful about the sunken cost fallacy. But also make sure that you're not just depressed for other reasons, because if that's the case trying to study something else won't do much.