“一定是我（你,他,她）…………”这句式用英文怎么表达

It must be myself/you/him/her麻烦举个例句Who did this? It must be him这是谁干的？一定是他那——"一定是我想多了"怎么说？I must be over thinking除了这样“I must be”还可以怎样译？I must be..... 我一定是.... 我得.... I must be leaving now 我必须得走了I must be dreaming 我一定在做梦I must be proud of myself 我必须为自个感到骄傲