If you think you can do a thing or think you can't do a thing, you're right.

Whether you think that you can, or that you can't, you are usually right.

If you think you can, or you think you can't, you’re right!

If you think you can, you can. And if you think you can't, you're right. also attributed to Mary Kay Ash

whether you think you can or you think you can't your right. Henry Ford

Even a mistake may turn out to be the one thing necessary to a worthwhile achievement.