Robert Griffin III and his wife have been separated for months -- after she found out he was dating a new woman ... and sources connected to the situation tell us she's absolutely devastated.
Sources close to the estranged couple tell us RG3 first met Florida State track star Grete Sadeiko through Instagram back in January ... roughly 7 months after his wife, Rebecca Liddicoat , gave birth to their daughter.
We're told the social media relationship turned real ... and when Rebecca found out about her in April, she was pissed.