Demi hit up an L.A. gym and returned home to a delicious surprise ... a cake with a message from her team ... "HAPPY 6 MO WE ARE SO F***ING PROUD OF YOU."

Demi has landed on her feet after her extended stint in rehab, following her near-fatal overdose last July.

She's been back in L.A. since November and has been dating her sober companion, Henry Levy.

Demi, who has struggled with addiction for years, was sober for 6 years before her relapse and has clearly made sobriety her priority. Several years ago she chose to live in a sober house for an extended period of time to help ensure she would not relapse.