I agree that stoking yourself up with positive affirmations and determination works for overcoming a difficult obstacle or achieving a particularly daunting task. I've done it myself. And that's fine for the little engine to make it up and over the hill.
It works, but it is definitely for the short-term. There will be more hills. You need to make it over the next one. And the next one. Every day. So you need to keep stoking yourself up, again and again, to face each hill and hardship as it comes. At some point, though, you're just going to run out of steam and your positive attitude will not be enough to make it up the hill.
Here's what I really think about the "you can do it if you believe you can" attitude. It's wishful thinking and shamanism. Most of a so-called positive attitude is smoke and vapor. It lacks content. It lacks a solid foundation. That's why you have to keep stoking it with daily positive affirmations. Without constant re-affirmations there's nothing to it.
What then is the right attitude? First of all, it's a state of mind, not a feeling. Most gurus will tell you that. But unlike what they tell you, it's not based on wishful thinking and positive speaking. It's based on who you really are and what you really want to achieve.
As I've written many times, you have to start with yourself, your core values and priorities. This is your sure and solid foundation. This is what you build and stand on to get through the daily grind and the tough times. There is nothing puffed-up, superficial or artificial about it. Build your attitude on this and you will make it over every hill, every day.