Chris Brown -- Jail Is Terrible ... I Feel Like a 'Caged Animal'

Chris Brown says he's finally been scared straight -- because he learned pretty quickly ... jail sucks.

You'd think the photos of Rihanna's battered face would have left an impression ... on him. But apparently it took a few days in solitary to shake Chris to the core.

One source -- who recently spoke with Brown on the phone -- tells us, Chris says he feels like "a caged animal," calling it "the worst experience of my life."

We're told Chris has been spending his time behind bars working out and writing music. He's telling people on the phone, "I'll never f*** up again."

Not particularly convincing.

Lucky for Chris ... he's rich enough to buy his way out of jail -- as we reported, he's negotiating to pay off his alleged assault victim in D.C. so the case goes away and the judge in the Rihanna case lets him loose ... to roam freely again.