Amber Portwood's Jailhouse Letter -- Cops Are Afraid I'll Shiv Someone

Police don't even trust Amber Portwood with a pen -- this according to a letter the "Teen Mom" wrote from inside jail, claiming cops are only willing to give her a special flexible writing utensil ... designed to prevent stabbings.

Prisoner-proof pens are pretty standard issue inside the clink ... but according to Amber's letter -- obtained by TMZ -- she's still freaked out people think she's a danger to society ... and she'll end up in prison.

Amber writes, "I don't think I'll ever take this sh*t for granted. I'm really scared I'm going to prison ... All I think about is Leah and how she needs a ro-model (sic)."

She adds, "I am a good person and I pray that I can change my life."

Amber wrote the letter to her brother last month -- after she was thrown in an Indiana jail for drug possession and failing to provide a urine test (a stipulation of her probation).