It's been very cold here in Chicago and my 2013 mini Cooper take a long time to start and after driving for 15 minutes I shut it off to fuel up and again it was hard to start

It's really hard to say why it's "hard to start" the car -- cold or not -- without performing testing to see how the air intake, fuel delivery, ignition spark and emissions controls of the vehicle are operating. Assuming the battery is fully charged and able to handle the cold, I'd focus on the engine performance areas to look at.