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.”

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.”