"Something turns in on itself."...?

[Mehrgan]

Hi there,
Would anyone please tell me what turn in (on) exactly means in the following sentence?

Bottling up our rage, rather than expressing it in a healthy way, can trap feelings deep inside where they can fester and turn in on themselves.



Thanks.

 

[nyota]

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I'm not a teacher
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turn in (up)on oneself - to become introverted.
Over the years, she had turned in upon herself and was quiet and alone. The death of his wife caused Jed to turn in on himself.
The Free Dictionary

turn in on yourself - to start to think only about yourself and your own problems and to forget about other people and their problems.
MacMillan


To me it means they cumulate and possibly backfire at you.