How do I keep going for training?

I just quit a 16 mile run 3 miles in, there's no excuse, I just can't handle the constant discomfort. I'm 215lbs, 5'11, so not the fittest, my longest run is only 13.16 miles, and my full marathon is in 5 weeks. I really really want to finish it, and I know I probably can, but the fact that the long runs are so miserable just demotivates me so much, and it's also raining during this particular run. How do you guys find the drive to just keep going during those long runs?

Edit: Update: I did 12 miles today and felt really great! (Treadmill tho because the rain)