Savannah Chrisley's fiancé expressed regret for his past crimes in a letter to the judge right before sentencing in his fraud case.

We've obtained Jason Galanis' letter to the judge, submitted about 2 weeks before he was sentenced to 189 months in federal prison ... and he writes that he is "profoundly sorry" to the people he hurt -- and for not displaying a strong enough moral character.

Galanis says he's had a lot of time to think over the previous few years -- and he realized he wasn't sorry for the right reasons -- not fully realizing how badly he hurt some people.

He writes, "I no longer let possessions define me as I once did. At 50 years old, I live in a 80 square foot cell ... with a bunkbed, a stranger, a shared toilet and where I've eaten every meal since March."

Jason goes on to say he shouldn't have blamed his father -- John Galanis -- for his own failings ... but he promises to make it up to his old man. He adds he knows he can and will change.

Of course ... it's unclear how much this letter actually did to help him. Galanis was still sentenced to his long prison terms -- which only ended when President Donald Trump commuted his sentence last year.

Galanis pled guilty to 7 charges for a pair of financial fraud schemes ... including one involving a Native American tribe. He was housed at the same prison as Todd Chrisley -- and later met Savannah, with whom he hit it off.