Trace Adkins -- Strung Out in Rehab

Rehabbing country singer Trace Adkins is back under the influence ... of music -- TMZ has learned, he just bought a sweet new guitar in Florida to write music as part of his treatment.

As we reported, Trace fell off the wagon earlier this month on a country music cruise after 12 years of sobriety -- drunkenly confronting a Trace Adkins impersonator ... and quickly got himself into a treatment facility.

Sources tell us, Trace is currently enrolled in a Florida rehab center ... and his therapist encouraged him to keep writing songs in order to stay focused on positive things.

We're told Trace thought it was a great idea ... and on Saturday, he was escorted by rehab employees to a music store in Lake Worth, FL ... to purchase a used Martin guitar for $1,100.

Rehab never sounded so good.