NSAttributedString font resets after setting it to another selection, why?

Question

I am trying to make the selected text in a UITextview bold or italic and this works for me, but let's say I got a string "one two" and I make "one" bold, so it will look like this: "one two". Now if I want to make "two" bold or italic as well this happens: "one two". Is there any explanation why this happens and is there a fix for this?

let main_string = textView2.text
let string_to_change = selectedTextWBU
let range = (main_string! as NSString).range(of: string_to_change)
let attributedString = NSMutableAttributedString(string: main_string!, attributes: [kCTFontAttributeName as NSAttributedStringKey : font])

if selectedTextWBU == ""{

}else{

    //highlight
    if sender.tag == 1{
        print("SELECTEDTEXTWBU ==  \(selectedTextWBU)")
        textView2.attributedText = attributedString

        //italic
    }else if sender.tag == 2{
        attributedString.addAttribute(kCTFontAttributeName as 
            NSAttributedStringKey, value: italicsFont, range: range)          
        textView2.attributedText = attributedString

        //bold
    }else if sender.tag == 3{
        attributedString.addAttribute(kCTFontAttributeName as NSAttributedStringKey, value: boldFont, range: range)
        textView2.attributedText = attributedString

    }

}