its true, running is almost purely mental. but i found the way i am able to continue is by taking things a little at a time.
in high school i was forced to do 12 minute runs, where you had to run 12 minutes and record how many laps you could do. when you look at it beforehand, it seems a daunting task that is next to impossible. But if you take each minute at a time and set little goals for yourself, before you know it you have 1 minute left and you find yourself sprinting
moral of the story/my advice: break down your mile bit by bit, focus on running for another minute... another minute... and before you know it BOOM! 1 mile