My ex left me because I was talking to other girls online (sexting etc) I still love her and I know what I did was wrong but she said the person she fell in love with died and I’m just a stranger?

Hello. It looks like you’re looking for some sort of closure. The words she left you with are a little metaphoric but quite seem quite succinct. it looks like she’s saying that you are not the ‘better’ person she thought you were, or used to be, in the beginning of the relationship; and in that sense, the person she realises you are is a stranger to her previous perception of you. Through some of your actions such as sexting others online, she has discovered that you do not value and respect monogamous commitment towards her as much as she thought you did. We all make mistakes and sometimes the best outcome is just accepting the consequences and making different choices next time.