Believing you can do it is an essential part of your fitness journey. When you think as yourself as someone fit and strong you are more likely to accomplish what you want.
I came across two different studies showing that mental strength helps you doing more*. It's an easy thing to say that if you want it you can do it but I truly believe that when your mind is on a willing mode your body will follow.
Nobody is saying that if you didn't succeed it's because you didn't want it. It's more about how you see yourself and how much you believe in the possibility of achieving your goals. And your goals aren't the ones of your neighbours. We all are at different stages of our fitness process and our objectives need to suit each one of us.
Anyway the way you think is very important when you're exercising and I learned from yoga that smiling and relaxing your face make any physical effort better. It works for anything in life too :)
* Published in Shape Magazine on May 2015, study by Wayne Brewer TWU and on June 2015, study by the University Of British Columbia saying that "simply believing that you rule your body's fitness fate can motivate you to work out".