ضَرَبْتَ أَنْتَ نَفْسَكَ — You struck yourself.
ضَرَبْتَ: past verb, attached subject -ta.
أَنْتَ: detached pronoun reinforcing the subject.
نَفْسَكَ: accusative direct object of ḍarabta, with -ka as mudāf ilayh.
The word nafsaka is not semantic emphasis in this sentence.
When vowel marks are absent, case and meaning resolve the parsing. Do not confuse emphasis with the object.