The couple met and fell in love while they were both seeking treatment at Utah's Cirque Lodge rehab retreat this summer.
Sources tell People magazine the pair decided to end the romance at the beginning of this week.
A friend of the actress tells the publication, "(Lindsay) is concentrating on her life and her career."
Recent reports suggest the Mean Girls star is keen to return to the recording studio to work on her third album over Christmas, and she's rumored to be making a guest appearance on TV hit Ugly Betty when the Hollywood writers strike is resolved.
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