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(TMZ) - Robin Thicke was shockingly candid as he arrived in Washington D.C. Thursday afternoon ... making it clear he's not throwing in the towel on his marriage, telling a TMZ photog, "I'm just trying to get her back."

TMZ broke the story ... Paula Patton called him last Friday and said their marriage was over, and Robin flew to Canada -- where she was filming -- in a desperate but failed attempt to change her mind.  

As TMZ reported ... things really fell apart after Robin simulated sex with Miley Cyrus onstage at the VMAs last month.  But Robin told TMZ's photog, the performance had nothing to do with his marital problems.

Sources tell TMZ Paula was livid and felt "utterly disrespected" by Robin's antics with Miley.

Robin is in D.C. for a concert Thursday night.  He says his performance will be the same, even though some of his songs are about Paula.

He told the TMZ photog he's just trying to figure things out.

Photo Credit Getty Images