I read an article recently on Logline by U.K. based screenwriter Dylan Costello about how he motivates himself to write and it's basically this: he's made a conscious decision to view himself as a writer and therefore he is a writer. He sets aside at least one hour a day to write, no matter what. Even if what he writes is rubbish, he's still written something that could possibly be used for some future project.
I think this is excellent advice and I'd go further: IMHO, you have to view the writing as your real work and the other role, office manager or whatever, as what you do to pay the bills and nothing else. Disengage and do the work, but don't put your heart and soul into it. Save that for the writing, your real job!