'Teen Mom' Mackenzie McKee -- I Crashed My Brand New Car ... And I Blame Diabetes

"Teen Mom 3" and "16 and Pregnant" star Mackenzie McKee blames diabetes for an accident that landed her brand spankin' new car at the bottom of a ditch.

According to sources close to McKee ... the MTV star was driving on an Oklahoma highway with sky high blood sugar Saturday night, when she zoned out and suddenly swerved off the road ... totaling the 2013 Kia Sorento she bought the day before.

Good news is the car passed the crash test -- all the airbags deployed and Mackenzie wasn't hurt. Her 2 kids were not with her.

Now the painful part ... sources say McKee's on the hook for the entire $26k she dropped on the car because she didn't have insurance (she got a ticket for that too). We're told she planned to call her insurance company Monday to get coverage.

Decision making is not necessarily her strong point.