Britney Spears is in rehab for substance abuse ... TMZ has learned. She checked in a few days ago somewhere in the United States. Sources with direct knowledge tell TMZ ... people close to the singer had been urging her to check herself in and she finally agreed to do so voluntarily. As one source put it ... "She realizes she hit rock bottom." Britney has had issues with drugs -- in particular Adderall -- and alcohol for years. During her many trips to Mexico, she made it a point to replenish