The teacher finally told me: "Forget it, it doesn't matter," and I felt so stupid.

forget 是动词，意思是"忘记、不再去想"。 it 是代词，指代前面提到的事情或情况。 forget it 是一个固定短语，意思是"算了、别在意"，用于表示某事不重要或不必再提。