Robin Thicke -- Pours Out Heart On Stage ... I'm Lost Without Paula Patton

Robin Thicke got back to performing tonight and wasted no time using the stage to beg for Paula Patton's mercy ... opening up to the crowd about the break-up and dedicating a song to her.

Thicke wore his wedding ring through the show in Fairfax, VA and told the crowd -- "For y'all that don't know me and my wife separated, but I'm trying to get my girl back. She's a good woman."

He also told fans he'd written the song "Lost Without U" for Paula ... before launching into the hit -- which we gotta say is kinda the perfect song for a guy trying to get back on a chick's good side.