whenever I am sucking air going up a hill and my legs feel ready to give out I just tell myself - "you are not in abject, crushing pain in the throes of death, all you are really feeling is a little discomfort - in the entire scope of your life this is only a measley 5 minutes (10, 20, 1/2 hour - whatever) Suck it up and get tough.
Once I put my discomfort into perspective it lessens considerably. (and then I say "why the H$!! do I do this
It's about the journey and being in the moment, not about the destination