The former teen idol was playing a gig in Agoura (near L.A.) Saturday night, where he looked incredibly intoxicated throughout the show ... ultimately leading to a hard fall over a monitor.

David slurs through various songs and and takes long pauses, often forgetting lyrics. He said on stage that Sunday night is his final concert ever due to health reasons ... and he's retiring.

David has been to rehab several times and has struggled with alcoholism.

We've reached out to Cassidy's rep ... so far, no word back.