D. Martyn Lloyd-Jones wrote:
'Take no thought for the morrow,' means 'Do not be guilty of anxious thoughts about the morrow'. It does not mean that you do not take any thought at all, otherwise the farmer would not plough and harrow and sow. He is looking to the future, but he does not spend the whole of his time wondering and worrying about the end results of his work. No, he takes reasonable thought and then he leaves it.
Here again the whole question is where to draw the line. Thinking is right up to a point, but if you go beyond that point it becomes worry and anxiety and it paralyzes and cripples. In other words, although it is very right to think about the future, it is very wrong to be controlled by it.
The difficulty with people who are prey to these fears is that they are controlled by the future, they are dominated by thoughts of it, and there they are wringing their hands, doing nothing, depressed by fears about it. In fact, they are completely governed and mastered by the unknown future, and that is always wrong. To take thought is right, but to be controlled by the future is all wrong.
Spiritual Depression, 98.