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Codeine is constipating, so that's the likely cause of your problem.
There is no good way to soften your hard stool, but there are ways to help it to pass.
I would suggest a Fleet's enema, which you can get at any drugstore. That will soften the stool up somewhat, and help it to pass.
You may or may not need something to soften your stools after that, such as milk of magnesia. Best regards.
The enema should work. That's what I would do.