You live in a world where everyone wants shortcuts mate. I mean, they are trying to find a magic diet pill to help with the over weight and obesity issue. Finding a dedicated person is hard. BUT anyone can change, and that is the true job of a personal trainer IMO.
To instill in a person the courage, the desire, the hope, to change their ways, and become a better person, physically, mentally, and emotionally. As a personal trainer you will also end up being a cheap psychologist
Its like any leadership or counseling position. It is your job to help a person better themselves. But they must first be willing to help themselves, if you are to be able to help them.
Do it for the joy of seeing those ones who really latch on, and transform their lives. A few of those, and you just might be hooked. have complete faith in each client, even if they are waning. Never give up and it just might be that little bit of faith and encouragement they needed to push through.
Once you have some successful clients who get to the point of being able to even step away from needing a personal trainer, you are successful as one. though you may need to find a new client, you can look at a person who has changed thier life, how they life it, and for the better, and know that your hand played a role in changing their life, and it will always mean something to them. And they usually recommend you to others, and are a walking sales ad for your work!