Luke 12: 25-26 - And which of you with taking thought can add to his stature one cubit? If ye then be not able to do that thing which is least, why take ye thought for the rest?
Do you see what Jesus said? I didn't, until just recently. He said that thinking ourselves into growing taller is that "which is least." The least thing? I don't know about you, but it sounds pretty large to me.
And then He said, that if we can't even do that, what makes us think we can do the rest? The rest being taking care of ourselves. The verses go on to say that we should consider the lilies of the field, how they are taken care of by God.
We fool ourselves when we think we are taking care of ourselves. It's only by the loving kindness of our Lord that we are clothed and fed. Don't be fooled.