You don't have to rub it in. I already feel bad enough as it is.
rub it in直译过来就是：“反复摩擦它的里面”，这个短语的正确意思是：“火上浇油，故意触人痛处”。因此，当美国人说"You don't have to rub it in. I already feel bad enough as it is."时，他/她要表达的意思就是:"Don't pile on. I've felt bad enough."、"You make it worse, and I am upset enough."。
Benjamin: I know the boss blame you again. If I were you, I would teach him a lesson.
Shirley: You don't have to rub it in. I already feel bad enough as it is.
①I didn't expect that you would laugh at me!
②Don't bother me and just leave me alone.
③Please get away from me if you come to see my jokes.
④Will you keep your mouth shut?
1. rub vt. 擦，摩擦
He rubbed and rubbed but couldn't seem to get clean.
When object rub against each other, static electricity is produced.
Gently rub into the skin until absorbed.
2. pile on 堆积……之上
She will pile on the agony when she desctibes her visits to the dentist.
Pile on more coal; the fire is going out.