“I don’t know if I can do this,” you say, but the fact that you voice that thought means you’re willing to try, and that’s the difference.
You didn’t say “I can’t,” and you’re willing to see if you can.
That’s how everything moves forward: by testing your personal limits to find what you can do. More often that not, it’s more than you would have expected.
You can climb that mountain after all, but first you had to say “I don’t know if I can do this” and start, to see how far you could go — how much of “this” was possible, and the next thing you know is that you can do more than you knew.
“Do or do not. There is no ‘try,’” Yoda said. But to try is to start doing, and when you give it a try, you often find you can finish, look back and say, “I did it. I didn’t know if I could, but it was worth a try, and I discovered I could.”