If something is worth doing, it's likely that obstacles are going to come up. Nothing is easy forever, there will be days when you stand there and look around and think to yourself 'I should just give up, there is no point!"
Pushing through adversity is something that must be dealt with, in some cases it's frequent. I was having a conversation with someone about their exercise routine and discovered that I have apparently become akin to some sort of bumper sticker saying "What would Chris Barnes do?" After asking that question to themselves the person in question set aside the issues troubling them during their workout and finished what they had set out to do. Another person this past week asked me about workouts and I've had a couple requests to be someone's trainer. I'm still a bit shocked, but flattered at this revelation. I never set out to become some sort of inspiration to others.
I maintain that I would be a poor trainer as the expectations I had of myself are not something that I would assume others had. Really the only advice I can give that people can take or leave as they see fit is to 'Push through it'. Feel like quitting 10 minutes into your workout? Got a stitch in your side while working your cardio? Don't want to drag yourself to the gym? Push through it.
In the end it's not what a trainer wants you to achieve that matters, it's what you want for yourself that matters. And make no mistake, you have to want it for yourself, whether it's the high-score in Donkey Kong, a fitness goal or the ability to win an eating contest, if you don't want it for yourself, no one else can make you want it enough to get it done.