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Diddy is taking responsibility for his actions ... telling the judge in his case he's sorry for all the hurt and pain he's caused others through his actions.

The rapper filed a four-page letter Thursday ahead of his Friday sentencing hearing ... beginning with the apology before admitting the last two years have been the hardest of his life -- and there is "no one to blame for my current reality and situation but myself." Document Launch Doc

Diddy says he's learned a lot since he was incarcerated a little more than a year ago ... explaining pain became his teacher -- and going on to say his "downfall was rooted in his selfishness."

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