How to remove a phrase from text widget? Using tkinter, python

Question

If I press the button it inserts a phrase in a textbox widget. When I press the button again I would like to remove the inserted phrase from the textbox. But I can't figure out how to do that.

...

premade_buttons_frame = tk.Frame(main_frame)
        premade_buttons_frame.pack()
        
        self.b1 = tkm.Button(premade_buttons_frame, text="Button text", padx=5, command=self.premade_phrase_b1)
        self.b1.grid(row=0, column=0)

...

def premade_phrase_b1(self):
        premade_phrase = "My phrase. "
        button_text = "Button text"
        
        #it the phrase already is inserted
        if self.b1.cget("text") == 'Used':
            #get all the text from the textbox
            textbox_text = self.symptom_textbox.get("1.0",'end-1c')
            #find the start index of the phrase
            start_index_of_string = textbox_text.find(premade_phrase)
            print("index_of_string", start_index_of_string)
            
            #try to delete the phrase
            self.symptom_textbox.delete(f"1.{start_index_of_string}", f"1.{len(premade_phrase)}")
            
            # set button text back to original text
            self.b1.configure(text=button_text)
        
        #insert the phrase in the textbox at the end
        elif self.b1.cget("text") == button_text:
            self.symptom_textbox.insert(tk.END, premade_phrase)
            #change button text to Used
            self.b1.configure(text="Used") 

If I only add the phrase it works. But If I type anything else in the textbox before trying to delete the phrase the delete will be messed and not deleting the added phrase (only parts of it).

How can I delete the inserted phrase (when pressing the button again after inserted the phrase)?