Why do I feel like it's my fault that she left me?

Well basically, when a girl leaves you, why do you feel like you didn't do enough for her?

Like you either over did the love showing or that you didn't go on enough dates or that you didn't treat her well. Despite her saying you show your love a lot to me, and that we don't need to go on fancy dates just keep it basic. But then the very next week leaves you because she was not happy in the relationship.
Why do I feel like it's my fault that she left me?
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